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/activities.py

@@ -4,23 +4,81 @@ Activities run inside a Worker bound to 'orchestrator-tq'.
 Each function is decorated with @activity.defn and executed by
 RunActivityWorkflow via workflow.execute_activity().
 
-Implementations are added in T14–T17; stubs here let the worker
-register correctly before the bodies are written.
+DB access pattern: worker.py calls init_session_factory(url) once before
+starting workers, which sets the module-level _session_factory used by
+activities that need DB access.
 """
 
 from __future__ import annotations
 
-from temporalio import activity
+import uuid
 
+from sqlalchemy import select
+from sqlalchemy.ext.asyncio import AsyncSession, async_sessionmaker
+from temporalio import activity
+from temporalio.exceptions import ApplicationError
+
+from activity_core.db import make_engine
+from activity_core.orm import ActivityDefinition as ActivityDefinitionRow
+
+
+_session_factory: async_sessionmaker[AsyncSession] | None = None
+
+
+def init_session_factory(url: str) -> None:
+    """Initialise the shared DB session factory.
+
+    Must be called once from worker.py before workers are started.
+    """
+    global _session_factory
+    _session_factory = async_sessionmaker(make_engine(url), expire_on_commit=False)
+
+
+def _get_session_factory() -> async_sessionmaker[AsyncSession]:
+    if _session_factory is None:
+        raise RuntimeError(
+            "DB session factory not initialised — call init_session_factory() first"
+        )
+    return _session_factory
+
+
+# ── Activities ─────────────────────────────────────────────────────────────────
 
 @activity.defn
 async def load_activity_definition(activity_id: str) -> dict:
-    """Load an ActivityDefinition row from the DB by ID.
+    """Load an ActivityDefinition row from Postgres by ID.
 
-    Returns a JSON-serialisable dict (Pydantic .model_dump()).
-    Implemented in T14.
+    Returns a JSON-serialisable dict suitable for passing between
+    Temporal workflow steps.
+
+    Raises:
+        ApplicationError (non-retryable): if no row exists for activity_id.
     """
-    raise NotImplementedError("T14")
+    Session = _get_session_factory()
+    async with Session() as session:
+        row = await session.scalar(
+            select(ActivityDefinitionRow).where(
+                ActivityDefinitionRow.id == uuid.UUID(activity_id)
+            )
+        )
+
+    if row is None:
+        raise ApplicationError(
+            f"ActivityDefinition {activity_id!r} not found",
+            non_retryable=True,
+        )
+
+    return {
+        "id": str(row.id),
+        "name": row.name,
+        "enabled": row.enabled,
+        "trigger_type": row.trigger_type,
+        "trigger_config": row.trigger_config,
+        "context_sources": row.context_sources,
+        "task_templates": row.task_templates,
+        "dedupe_key_strategy": row.dedupe_key_strategy,
+        "version": row.version,
+    }
 
 
 @activity.defn

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(

workplans/custodian-WP-0001-temporal-backbone.md

@@ -60,7 +60,7 @@ tasks:
     state_hub_task_id: e0205c56-1d40-4142-952b-e27ff6a44e1d
   - id: T14
     title: Implement load_activity_definition activity
-    status: todo
+    status: done
     state_hub_task_id: b05f046f-a6ba-4d96-a298-a0bbea067427
   - id: T15
     title: Implement resolve_context activity (stub)