feat(activities): implement load_activity_definition — T14

activities.py:
- init_session_factory(url): module-level async_sessionmaker init,
  called once from worker.py before workers start
- load_activity_definition(activity_id): queries activity_definitions
  by UUID, returns JSON-serialisable dict; raises ApplicationError
  (non_retryable=True) if row not found

worker.py:
- reads ACTCORE_DB_URL at startup, fails fast if missing
- calls init_session_factory() before connecting to Temporal

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/activity_core/worker.py

@@ -24,6 +24,7 @@ from temporalio.client import Client
 from temporalio.worker import Worker
 
 from activity_core.activities import (
+    init_session_factory,
     load_activity_definition,
     log_run,
     resolve_context,
@@ -38,6 +39,11 @@ TASK_EXECUTION_TASK_QUEUE = "task-execution-tq"
 
 
 async def run() -> None:
+    db_url = os.environ.get("ACTCORE_DB_URL")
+    if not db_url:
+        raise RuntimeError("ACTCORE_DB_URL is required")
+    init_session_factory(db_url)
+
     client = await Client.connect(TEMPORAL_HOST, namespace=TEMPORAL_NAMESPACE)
 
     orchestrator_worker = Worker(