Phase 2.1-2.3: Add exchange endpoints

Add exchange trading endpoints:
- POST /api/exchange: Create exchange trade
  - Validates slot, price staleness, EUR amount limits
  - Calculates sats from EUR and agreed price
  - Direction-specific premium (buy=+5%, sell=-5%)

- GET /api/trades: List user's exchanges
- POST /api/trades/{id}/cancel: Cancel user's exchange

Add schemas:
- ExchangeRequest, ExchangeResponse
- ExchangeUserContact, AdminExchangeResponse (for Phase 2.4)
- PaginatedExchanges, PaginatedAdminExchanges

backend/schemas.py

@@ -234,6 +234,74 @@ class PriceHistoryResponse(BaseModel):
     created_at: datetime
 
 
+# =============================================================================
+# Exchange Schemas
+# =============================================================================
+
+
+class ExchangeRequest(BaseModel):
+    """Request to create an exchange trade."""
+
+    slot_start: datetime
+    direction: str  # "buy" or "sell"
+    eur_amount: int  # EUR cents (e.g., 10000 = €100)
+
+
+class ExchangeResponse(BaseModel):
+    """Response model for an exchange trade."""
+
+    id: int
+    user_id: int
+    user_email: str
+    slot_start: datetime
+    slot_end: datetime
+    direction: str
+    eur_amount: int  # EUR cents
+    sats_amount: int  # Satoshis
+    market_price_eur: float
+    agreed_price_eur: float
+    premium_percentage: int
+    status: str
+    created_at: datetime
+    cancelled_at: datetime | None
+    completed_at: datetime | None
+
+
+class ExchangeUserContact(BaseModel):
+    """User contact info for admin view."""
+
+    email: str
+    contact_email: str | None
+    telegram: str | None
+    signal: str | None
+    nostr_npub: str | None
+
+
+class AdminExchangeResponse(BaseModel):
+    """Response model for admin exchange view (includes user contact)."""
+
+    id: int
+    user_id: int
+    user_email: str
+    user_contact: ExchangeUserContact
+    slot_start: datetime
+    slot_end: datetime
+    direction: str
+    eur_amount: int
+    sats_amount: int
+    market_price_eur: float
+    agreed_price_eur: float
+    premium_percentage: int
+    status: str
+    created_at: datetime
+    cancelled_at: datetime | None
+    completed_at: datetime | None
+
+
+PaginatedExchanges = PaginatedResponse[ExchangeResponse]
+PaginatedAdminExchanges = PaginatedResponse[AdminExchangeResponse]
+
+
 # =============================================================================
 # Meta/Constants Schemas
 # =============================================================================