Add user settings endpoint and exchange rate fallback APIs

Backend now stores user settings (dashboard config) in a JSONB column and
exposes CRUD via /settings/. Exchange rate service gains multiple fallback
providers (ExchangeRate-API, currency-api, FloatRates) so USD/CRC rates
stay available when BCCR is down.

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

backend/app/models/models.py

@@ -2,6 +2,8 @@ import enum
 from datetime import datetime
 from typing import Optional
 
+from sqlalchemy import Column
+from sqlalchemy.dialects.postgresql import JSONB
 from sqlmodel import Field, Relationship, SQLModel
 
 
@@ -195,3 +197,26 @@ class APITokenRead(SQLModel):
     created_at: datetime
     expires_at: Optional[datetime]
     is_active: bool
+
+
+# --- User Settings ---
+
+
+class UserSettings(SQLModel, table=True):
+    id: Optional[int] = Field(default=None, primary_key=True)
+    key: str = Field(index=True, unique=True, default="default")
+    data: dict = Field(
+        default_factory=dict,
+        sa_column=Column(JSONB, nullable=False, server_default="{}"),
+    )
+    updated_at: datetime = Field(default_factory=datetime.utcnow)
+
+
+class UserSettingsRead(SQLModel):
+    key: str
+    data: dict
+    updated_at: datetime
+
+
+class UserSettingsUpdate(SQLModel):
+    data: dict