feat: add PostgresModelUsageStore and ModelUsageTracker.seed()

- Add seed() method to ModelUsageTracker for bulk-loading persisted entries at startup
- Create PostgresModelUsageStore for persistence of model usage counters to Postgres
- Store only current cumulative snapshots (no historical time-series)
- Use standard CREATE TABLE IF NOT EXISTS idiom matching other Postgres stores
- Add comprehensive mocked unit tests for both components

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

backend/app/shared/model_usage_tracker.py

@@ -97,6 +97,16 @@ class ModelUsageTracker:
         except Exception as exc:  # noqa: BLE001 - tracking must never break a real call
             logger.warning("ModelUsageTracker.record failed for {}:{} - {}", provider, model, exc)
 
+    def seed(self, entries: dict[str, ModelUsageEntry]) -> None:
+        """Bulk-load persisted entries (called once at startup, before any traffic).
+
+        Unlike record(), this replaces entries wholesale rather than
+        accumulating deltas — it exists to restore counters saved by a
+        previous process run, not to record a new call.
+        """
+        with self._lock:
+            self._entries.update(entries)
+
     def snapshot(self) -> dict[str, ModelUsageEntry]:
         """Return a shallow copy of all tracked entries, safe to mutate by the caller."""
         with self._lock: