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/routes/public.py

@@ -0,0 +1,48 @@
+"""Public endpoints: price transparency data (no auth required)."""
+
+from typing import Annotated
+from uuid import UUID
+
+from fastapi import APIRouter, Depends, HTTPException, Query, status
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from cartsnitch_api.database import get_db
+from cartsnitch_api.schemas import (
+    PublicInflationResponse,
+    PublicStoreComparisonResponse,
+    PublicTrendResponse,
+)
+from cartsnitch_api.services.public import PublicService
+
+router = APIRouter(prefix="/public", tags=["public"])
+
+
+@router.get("/trends/{product_id}", response_model=PublicTrendResponse)
+async def public_price_trend(product_id: UUID, db: AsyncSession = Depends(get_db)):
+    svc = PublicService(db)
+    try:
+        return await svc.get_trend(product_id)
+    except LookupError:
+        raise HTTPException(
+            status_code=status.HTTP_404_NOT_FOUND, detail="Product not found"
+        ) from None
+
+
+@router.get("/store-comparison", response_model=PublicStoreComparisonResponse)
+async def public_store_comparison(
+    product_ids: Annotated[list[UUID], Query(max_length=20)],
+    db: AsyncSession = Depends(get_db),
+):
+    if not product_ids:
+        raise HTTPException(
+            status_code=status.HTTP_400_BAD_REQUEST,
+            detail="At least one product_id is required",
+        )
+    svc = PublicService(db)
+    return await svc.get_store_comparison(product_ids)
+
+
+@router.get("/inflation", response_model=PublicInflationResponse)
+async def public_inflation(db: AsyncSession = Depends(get_db)):
+    svc = PublicService(db)
+    return await svc.get_inflation()