feat(db): ORM models + Alembic migrations 0001–0003 — T09/T10/T11

SQLAlchemy ORM (src/activity_core/orm.py):
  - ActivityDefinition, ActivityRun, TaskInstance mapped to Base.metadata
  - Wired into migrations/env.py for autogenerate support

Migrations (chained 0001 → 0002 → 0003):
  - 0001: activity_definitions (id, name, enabled, trigger_type,
          trigger_config JSONB, context_sources JSONB, task_templates JSONB,
          dedupe_key_strategy, version, created_at, updated_at)
  - 0002: activity_runs (run_id, activity_id FK→activity_definitions,
          scheduled_for, fired_at, context_snapshot JSONB, tasks_spawned,
          version_used) + index on activity_id
  - 0003: task_instances (id, run_id FK→activity_runs CASCADE,
          type, params JSONB, status, created_at) + index on run_id

Apply with: ACTCORE_DB_URL=... alembic upgrade head

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

migrations/env.py

@@ -8,7 +8,8 @@ from sqlalchemy.ext.asyncio import async_engine_from_config
 
 from alembic import context
 
-from activity_core.db import Base  # noqa: F401 — imports all ORM models via submodules
+from activity_core.db import Base  # noqa: F401
+import activity_core.orm  # noqa: F401 — registers all ORM tables into Base.metadata
 
 # Alembic Config object — access to values in alembic.ini
 config = context.config