fix: normalize date-bucket keys for Postgres dashboard charts

Dashboard charts (activity, message-activity, node-count) rendered as
flat zeros on Postgres because func.date() returns a str on SQLite but
a datetime.date on Postgres — the dict lookup by string key always
missed. Fixed with a dialect-neutral _date_bucket_key() helper and
pinned the Postgres session timezone to UTC at the engine level.

Also adds dual-backend test infrastructure (TEST_DATABASE_BACKEND env
var), per-worker Postgres databases for pytest-xdist isolation, and
strengthened regression tests asserting non-zero date buckets.

tests/test_api/test_user_profiles.py

@@ -95,13 +95,23 @@ class TestListProfiles:
         api_db_session.add(adoption)
         api_db_session.commit()
 
-        api_db_session.execute(text("PRAGMA foreign_keys=OFF"))
+        # Temporarily disable FK enforcement to simulate an orphaned adoption
+        # (node deleted while the adoption record persists). SQLite uses
+        # PRAGMA; Postgres uses session_replication_role = replica.
+        dialect = api_db_session.bind.dialect.name  # type: ignore[union-attr]
+        if dialect == "postgresql":
+            api_db_session.execute(text("SET session_replication_role = replica"))
+        else:
+            api_db_session.execute(text("PRAGMA foreign_keys=OFF"))
         api_db_session.execute(
             text("DELETE FROM nodes WHERE id = :id"),
             {"id": sample_node.id},
         )
         api_db_session.commit()
-        api_db_session.execute(text("PRAGMA foreign_keys=ON"))
+        if dialect == "postgresql":
+            api_db_session.execute(text("SET session_replication_role = DEFAULT"))
+        else:
+            api_db_session.execute(text("PRAGMA foreign_keys=ON"))
 
         response = client_no_auth.get(
             "/api/v1/user/profiles",