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/tests/observability/test_model_usage_tracker.py

@@ -77,3 +77,33 @@ def test_snapshot_returns_independent_copy():
 def test_get_model_usage_tracker_returns_singleton():
     """get_model_usage_tracker() always returns the same process-wide instance."""
     assert get_model_usage_tracker() is get_model_usage_tracker()
+
+
+def test_seed_populates_registry_from_persisted_entries():
+    """seed() must bulk-load entries (e.g. from Postgres at startup) into the registry."""
+    tracker = ModelUsageTracker()
+    persisted = {
+        "deepseek:deepseek-v4-flash": ModelUsageEntry(
+            provider="deepseek", model="deepseek-v4-flash", total_tokens=500, call_count_ok=20,
+        ),
+    }
+
+    tracker.seed(persisted)
+
+    entry = tracker.get("deepseek", "deepseek-v4-flash")
+    assert entry.total_tokens == 500
+    assert entry.call_count_ok == 20
+
+
+def test_seed_then_record_accumulates_on_top_of_seeded_value():
+    """A call recorded after seeding must add to the seeded total, not replace it."""
+    tracker = ModelUsageTracker()
+    tracker.seed({
+        "deepseek:deepseek-v4-flash": ModelUsageEntry(
+            provider="deepseek", model="deepseek-v4-flash", total_tokens=500,
+        ),
+    })
+
+    tracker.record(provider="deepseek", model="deepseek-v4-flash", success=True, usage={"total_tokens": 10})
+
+    assert tracker.get("deepseek", "deepseek-v4-flash").total_tokens == 510