feat(db): SharedTableProtocol + Store.clone() for dual-backend support

app/db/protocol.py

@@ -0,0 +1,57 @@
+"""Protocol (duck-type interface) for shared table backends (SQLite and Postgres)."""
+from __future__ import annotations
+
+from typing import Optional, Protocol, runtime_checkable
+
+from app.db.models import MarketComp, Seller
+
+
+@runtime_checkable
+class SharedTableProtocol(Protocol):
+    """Protocol that both Store (SQLite) and SnipeSharedStore (Postgres) must satisfy.
+
+    This enables code that reads/writes shared tables (sellers, market_comps, reported_sellers)
+    to remain agnostic to the underlying backend.
+    """
+
+    def save_seller(self, seller: Seller) -> None:
+        """Persist a single seller record."""
+        ...
+
+    def save_sellers(self, sellers: list[Seller]) -> None:
+        """Persist multiple seller records (batch upsert)."""
+        ...
+
+    def get_seller(self, platform: str, platform_seller_id: str) -> Optional[Seller]:
+        """Fetch a single seller by platform and platform_seller_id."""
+        ...
+
+    def save_market_comp(self, comp: MarketComp) -> None:
+        """Persist a market comparison record."""
+        ...
+
+    def get_market_comp(self, platform: str, query_hash: str) -> Optional[MarketComp]:
+        """Fetch a market comparison by platform and query_hash."""
+        ...
+
+    def mark_reported(
+        self,
+        platform: str,
+        platform_seller_id: str,
+        username: Optional[str] = None,
+        reported_by: str = "user",
+    ) -> None:
+        """Record that a seller has been reported to the platform."""
+        ...
+
+    def list_reported(self, platform: str = "ebay") -> list[str]:
+        """Return all platform_seller_ids that have been reported."""
+        ...
+
+    def delete_seller_data(self, platform: str, platform_seller_id: str) -> None:
+        """Permanently erase a seller and all related data (GDPR/eBay compliance)."""
+        ...
+
+    def clone(self) -> SharedTableProtocol:
+        """Create a new independent instance pointing to the same backend."""
+        ...

app/db/store.py

@@ -21,6 +21,10 @@ class Store:
         # WAL mode: allows concurrent readers + one writer without blocking
         self._conn.execute("PRAGMA journal_mode=WAL")
 
+    def clone(self) -> Store:
+        """Create a new independent instance pointing to the same database."""
+        return Store(self._db_path)
+
     # --- Seller ---
 
     def delete_seller_data(self, platform: str, platform_seller_id: str) -> None:

tests/db/test_protocol.py

@@ -0,0 +1,16 @@
+"""Verify Store satisfies SharedTableProtocol at import time."""
+from app.db.protocol import SharedTableProtocol
+from app.db.store import Store
+
+
+def test_store_satisfies_protocol():
+    assert issubclass(Store, SharedTableProtocol)
+
+
+def test_store_clone_returns_new_instance(tmp_path):
+    db = tmp_path / "test.db"
+    s = Store(db)
+    clone = s.clone()
+    assert isinstance(clone, Store)
+    assert clone is not s
+    assert clone._db_path == db