feat(db): init Alembic (async) + SQLAlchemy declarative base — T08

- alembic init -t async migrations
- alembic.ini: dev fallback URL postgresql+asyncpg://…:5433/actcore;
  ACTCORE_DB_URL env var overrides at runtime; src/ added to sys.path
- migrations/env.py: reads ACTCORE_DB_URL, wires target_metadata to Base.metadata
- src/activity_core/db.py: DeclarativeBase subclass + make_engine() helper

Tool choice: Alembic + SQLAlchemy[asyncio] (already declared in pyproject.toml).
Migrations run with: ACTCORE_DB_URL=... alembic upgrade head

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

src/activity_core/db.py

@@ -0,0 +1,19 @@
+"""SQLAlchemy async engine and declarative base for activity-core."""
+
+import os
+
+from sqlalchemy.ext.asyncio import AsyncEngine, create_async_engine
+from sqlalchemy.orm import DeclarativeBase
+
+
+class Base(DeclarativeBase):
+    pass
+
+
+def make_engine(url: str | None = None) -> AsyncEngine:
+    """Return an async engine. Falls back to ACTCORE_DB_URL, then the dev default."""
+    resolved = url or os.environ.get(
+        "ACTCORE_DB_URL",
+        "postgresql+asyncpg://actcore:actcore@localhost:5433/actcore",
+    )
+    return create_async_engine(resolved, echo=False)