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/README

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+Generic single-database configuration with an async dbapi.

migrations/env.py

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+import asyncio
+import os
+from logging.config import fileConfig
+
+from sqlalchemy import pool
+from sqlalchemy.engine import Connection
+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
+
+# Alembic Config object — access to values in alembic.ini
+config = context.config
+
+# Set up loggers from alembic.ini
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
+
+# ORM metadata for autogenerate
+target_metadata = Base.metadata
+
+# Override the DB URL from env var when running live migrations
+_db_url = os.environ.get("ACTCORE_DB_URL")
+if _db_url:
+    config.set_main_option("sqlalchemy.url", _db_url)
+
+
+def run_migrations_offline() -> None:
+    """Generate SQL without a live DB connection (alembic upgrade --sql)."""
+    url = config.get_main_option("sqlalchemy.url")
+    context.configure(
+        url=url,
+        target_metadata=target_metadata,
+        literal_binds=True,
+        dialect_opts={"paramstyle": "named"},
+    )
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+def do_run_migrations(connection: Connection) -> None:
+    context.configure(connection=connection, target_metadata=target_metadata)
+    with context.begin_transaction():
+        context.run_migrations()
+
+
+async def run_async_migrations() -> None:
+    connectable = async_engine_from_config(
+        config.get_section(config.config_ini_section, {}),
+        prefix="sqlalchemy.",
+        poolclass=pool.NullPool,
+    )
+    async with connectable.connect() as connection:
+        await connection.run_sync(do_run_migrations)
+    await connectable.dispose()
+
+
+def run_migrations_online() -> None:
+    asyncio.run(run_async_migrations())
+
+
+if context.is_offline_mode():
+    run_migrations_offline()
+else:
+    run_migrations_online()

migrations/script.py.mako

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+"""${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"}