feat: merge cartsnitch/api into api/ subdirectory

Consolidate API gateway service into monorepo.
Squashed from https://github.com/cartsnitch/api main (89bacb1).

Co-Authored-By: Paperclip <noreply@paperclip.ing>

src/cartsnitch_api/services/alerts.py

@@ -0,0 +1,75 @@
+"""Alert service — price and shrinkflation alerts for users.
+
+Alerts are generated by StickerShock and ShrinkRay services and written to the DB.
+This service reads them for the API gateway.
+"""
+
+from uuid import UUID
+
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm import selectinload
+
+
+class AlertService:
+    def __init__(self, db: AsyncSession) -> None:
+        self.db = db
+
+    async def list_alerts(self, user_id: UUID) -> list[dict]:
+        """List shrinkflation events for products the user has purchased."""
+        from cartsnitch_api.models import Purchase, PurchaseItem, ShrinkflationEvent
+
+        # Get product IDs from user's purchases
+        items_result = await self.db.execute(
+            select(PurchaseItem.normalized_product_id)
+            .join(Purchase)
+            .where(
+                Purchase.user_id == user_id,
+                PurchaseItem.normalized_product_id.isnot(None),
+            )
+            .distinct()
+        )
+        product_ids = [row[0] for row in items_result.all()]
+
+        if not product_ids:
+            return []
+
+        result = await self.db.execute(
+            select(ShrinkflationEvent)
+            .where(ShrinkflationEvent.normalized_product_id.in_(product_ids))
+            .options(selectinload(ShrinkflationEvent.normalized_product))
+            .order_by(ShrinkflationEvent.detected_date.desc())
+        )
+        events = result.scalars().all()
+
+        return [
+            {
+                "id": e.id,
+                "alert_type": "shrinkflation",
+                "product_id": e.normalized_product_id,
+                "product_name": e.normalized_product.canonical_name,
+                "message": (
+                    f"Size changed from {e.old_size}{e.old_unit} to {e.new_size}{e.new_unit}"
+                ),
+                "triggered_at": e.detected_date,
+                "read": False,
+            }
+            for e in events
+        ]
+
+    async def get_settings(self, user_id: UUID) -> dict:
+        # Alert settings would be stored in a user_settings table.
+        # For now, return defaults since the table doesn't exist yet in common lib.
+        return {
+            "price_increase_threshold_pct": 5.0,
+            "shrinkflation_enabled": True,
+            "email_notifications": False,
+        }
+
+    async def update_settings(self, user_id: UUID, **fields) -> dict:
+        # Would update user_settings table. Return merged defaults for now.
+        current = await self.get_settings(user_id)
+        for k, v in fields.items():
+            if v is not None and k in current:
+                current[k] = v
+        return current