Error visibility and agent tool cleanup

Exchange-rate fetchers log every source failure instead of silently
passing (BE-18); the rate tool exposes fetched_at so staleness is
visible (BE-20). Agent: unbound-session failures raise a clear
RuntimeError (BE-12); net worth converts via get_crc_multipliers with
last-known fallbacks instead of a hardcoded 600 CRC / 1.08 EUR guess,
and reports accounts it cannot convert rather than inventing numbers
(BE-24); budget tool enforces MIN/MAX_YEAR (BE-16); municipal receipts
gain offset paging (BE-17); category analytics prefetches names; the
module docstring pins the read-only tool policy (SEC-09). Note: the
refresh loop never swallowed CancelledError (BaseException since 3.8) —
ARCH-08 was a false positive.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/app/services/exchange_rate.py

@@ -61,8 +61,8 @@ def _fetch_bccr_rate(indicator: int, date_str: str) -> float | None:
         for datos in root.iter():
             if datos.tag.endswith("NUM_VALOR"):
                 return float(datos.text.strip().replace(",", "."))
-    except Exception:
-        pass
+    except Exception as e:
+        logger.warning("BCCR rate fetch failed (indicator %s): %s", indicator, e)
     return None
 
 
@@ -91,8 +91,8 @@ def _fetch_exchangerate_api() -> tuple[float, float] | None:
             crc = data["rates"].get("CRC")
             if crc:
                 return _mid_to_buy_sell(float(crc))
-    except Exception:
-        pass
+    except Exception as e:
+        logger.warning("ExchangeRate-API fetch failed: %s", e)
     return None
 
 
@@ -106,7 +106,8 @@ def _fetch_currency_api() -> tuple[float, float] | None:
             crc = data.get("usd", {}).get("crc")
             if crc:
                 return _mid_to_buy_sell(float(crc))
-        except Exception:
+        except Exception as e:
+            logger.warning("currency-api fetch failed (%s): %s", url, e)
             continue
     return None
 
@@ -120,8 +121,8 @@ def _fetch_floatrates() -> tuple[float, float] | None:
         crc_data = data.get("crc")
         if crc_data and "rate" in crc_data:
             return _mid_to_buy_sell(float(crc_data["rate"]))
-    except Exception:
-        pass
+    except Exception as e:
+        logger.warning("FloatRates fetch failed: %s", e)
     return None
 
 
@@ -199,8 +200,8 @@ def _fetch_fiat_crc_mid(code: str) -> float | None:
             x = data["rates"].get(code)
             if crc and x:
                 return float(crc) / float(x)
-    except Exception:
-        pass
+    except Exception as e:
+        logger.warning("%s/CRC fiat rate fetch failed: %s", code, e)
     return None
 
 
@@ -220,8 +221,8 @@ def _fetch_crypto_crc(code: str) -> float | None:
         price = data.get(coin_id, {}).get("crc")
         if price:
             return float(price)
-    except Exception:
-        pass
+    except Exception as e:
+        logger.warning("CoinGecko %s/CRC fetch failed: %s", code, e)
     return None