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>

migrations/script.py.mako

@@ -0,0 +1,28 @@
+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+${imports if imports else ""}
+
+# revision identifiers, used by Alembic.
+revision: str = ${repr(up_revision)}
+down_revision: Union[str, Sequence[str], None] = ${repr(down_revision)}
+branch_labels: Union[str, Sequence[str], None] = ${repr(branch_labels)}
+depends_on: Union[str, Sequence[str], None] = ${repr(depends_on)}
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    ${upgrades if upgrades else "pass"}
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    ${downgrades if downgrades else "pass"}