Convert all currencies to CRC and poll rates every 6h

Budget/transactions/salarios totals summed Transaction.amount directly,
so USD/EUR entries were treated as CRC and effectively disappeared from
the dashboard (the analytics fix in 9a80f2a only covered analytics).
Adds a shared get_converted_amount_expr() helper driven by the full
Currency enum — USD/EUR via ExchangeRate-API, BTC/XMR via CoinGecko —
and wires it into every func.sum(Transaction.amount) site.

Also starts a background task in the FastAPI lifespan that force-refreshes
every currency 4x/day, persisting USD to the DB and updating in-memory
caches for the rest. Failures are swallowed per-currency so a CoinGecko
outage cannot take out USD/EUR, and the last-known rate is always retained.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/budget_projection.py

@@ -12,6 +12,7 @@ from app.models.models import (
     TransactionSource,
     TransactionType,
 )
+from app.services.exchange_rate import get_converted_amount_expr
 
 MIN_YEAR = 2026
 MAX_YEAR = 2030
@@ -104,13 +105,15 @@ def compute_actuals_by_source(
     prev_start, prev_end = get_cycle_range(prev_cc_y, prev_cc_m)
     cal_start, cal_end = get_month_range(year, month)
 
+    amount_crc = get_converted_amount_expr(session)
+
     results = {}
     for source in TransactionSource:
         if source == TransactionSource.CREDIT_CARD:
             start, end = cc_start, cc_end
             # Normal transactions in this cycle (not deferred)
             compra_normal = session.exec(
-                select(func.coalesce(func.sum(Transaction.amount), 0)).where(
+                select(func.coalesce(func.sum(amount_crc), 0)).where(
                     Transaction.date >= start,
                     Transaction.date < end,
                     Transaction.source == source,
@@ -120,7 +123,7 @@ def compute_actuals_by_source(
             ).one()
             # Deferred from previous cycle
             compra_deferred = session.exec(
-                select(func.coalesce(func.sum(Transaction.amount), 0)).where(
+                select(func.coalesce(func.sum(amount_crc), 0)).where(
                     Transaction.date >= prev_start,
                     Transaction.date < prev_end,
                     Transaction.source == source,
@@ -131,7 +134,7 @@ def compute_actuals_by_source(
             compra = float(compra_normal) + float(compra_deferred)
 
             dev_normal = session.exec(
-                select(func.coalesce(func.sum(Transaction.amount), 0)).where(
+                select(func.coalesce(func.sum(amount_crc), 0)).where(
                     Transaction.date >= start,
                     Transaction.date < end,
                     Transaction.source == source,
@@ -140,7 +143,7 @@ def compute_actuals_by_source(
                 )
             ).one()
             dev_deferred = session.exec(
-                select(func.coalesce(func.sum(Transaction.amount), 0)).where(
+                select(func.coalesce(func.sum(amount_crc), 0)).where(
                     Transaction.date >= prev_start,
                     Transaction.date < prev_end,
                     Transaction.source == source,
@@ -180,7 +183,7 @@ def compute_actuals_by_source(
         else:
             # Cash / Transfer: calendar month, no deferred logic
             compra = session.exec(
-                select(func.coalesce(func.sum(Transaction.amount), 0)).where(
+                select(func.coalesce(func.sum(amount_crc), 0)).where(
                     Transaction.date >= cal_start,
                     Transaction.date < cal_end,
                     Transaction.source == source,
@@ -188,7 +191,7 @@ def compute_actuals_by_source(
                 )
             ).one()
             devolucion = session.exec(
-                select(func.coalesce(func.sum(Transaction.amount), 0)).where(
+                select(func.coalesce(func.sum(amount_crc), 0)).where(
                     Transaction.date >= cal_start,
                     Transaction.date < cal_end,
                     Transaction.source == source,
@@ -230,6 +233,8 @@ def compute_actuals_by_category(
     prev_start, prev_end = get_cycle_range(prev_cc_y, prev_cc_m)
     cal_start, cal_end = get_month_range(year, month)
 
+    amount_crc = get_converted_amount_expr(session)
+
     totals: dict[int, float] = {}
 
     def _merge_rows(rows: list) -> None:
@@ -245,7 +250,7 @@ def compute_actuals_by_category(
             select(
                 Transaction.category_id,
                 Transaction.transaction_type,
-                func.sum(Transaction.amount),
+                func.sum(amount_crc),
             )
             .where(
                 Transaction.date >= cc_start,
@@ -265,7 +270,7 @@ def compute_actuals_by_category(
             select(
                 Transaction.category_id,
                 Transaction.transaction_type,
-                func.sum(Transaction.amount),
+                func.sum(amount_crc),
             )
             .where(
                 Transaction.date >= prev_start,
@@ -285,7 +290,7 @@ def compute_actuals_by_category(
             select(
                 Transaction.category_id,
                 Transaction.transaction_type,
-                func.sum(Transaction.amount),
+                func.sum(amount_crc),
             )
             .where(
                 Transaction.date >= cal_start,
@@ -310,6 +315,8 @@ def compute_cc_by_category(
     prev_cc_y, prev_cc_m = get_previous_cycle(cc_cycle_y, cc_cycle_m)
     prev_start, prev_end = get_cycle_range(prev_cc_y, prev_cc_m)
 
+    amount_crc = get_converted_amount_expr(session)
+
     totals: dict[int | None, float] = {}
 
     def _merge(rows: list) -> None:
@@ -325,7 +332,7 @@ def compute_cc_by_category(
             select(
                 Transaction.category_id,
                 Transaction.transaction_type,
-                func.sum(Transaction.amount),
+                func.sum(amount_crc),
             )
             .where(
                 Transaction.date >= cc_start,
@@ -343,7 +350,7 @@ def compute_cc_by_category(
             select(
                 Transaction.category_id,
                 Transaction.transaction_type,
-                func.sum(Transaction.amount),
+                func.sum(amount_crc),
             )
             .where(
                 Transaction.date >= prev_start,
@@ -449,6 +456,8 @@ def compute_monthly_projection(
     prev_start, prev_end = get_cycle_range(prev_cc_y, prev_cc_m)
     cal_start, cal_end = get_month_range(year, month)
 
+    amount_crc = get_converted_amount_expr(session)
+
     def _sum_uncategorized(rows: list) -> float:
         total = 0.0
         for tx_type, amount in rows:
@@ -461,7 +470,7 @@ def compute_monthly_projection(
     # CC uncategorized: this cycle (not deferred)
     uncovered_actual += _sum_uncategorized(
         session.exec(
-            select(Transaction.transaction_type, func.sum(Transaction.amount))
+            select(Transaction.transaction_type, func.sum(amount_crc))
             .where(
                 Transaction.date >= cc_start,
                 Transaction.date < cc_end,
@@ -476,7 +485,7 @@ def compute_monthly_projection(
     # CC uncategorized: deferred from previous cycle
     uncovered_actual += _sum_uncategorized(
         session.exec(
-            select(Transaction.transaction_type, func.sum(Transaction.amount))
+            select(Transaction.transaction_type, func.sum(amount_crc))
             .where(
                 Transaction.date >= prev_start,
                 Transaction.date < prev_end,
@@ -491,7 +500,7 @@ def compute_monthly_projection(
     # Non-CC uncategorized: calendar month
     uncovered_actual += _sum_uncategorized(
         session.exec(
-            select(Transaction.transaction_type, func.sum(Transaction.amount))
+            select(Transaction.transaction_type, func.sum(amount_crc))
             .where(
                 Transaction.date >= cal_start,
                 Transaction.date < cal_end,