feat: add alembic migrations and PostgreSQL CI support

- Add alembic dependency and initialize migration framework
- Configure async alembic env.py for SQLAlchemy 2.0
- Create initial migration for PostORM table
- Gate init_db() with SKIP_INIT_DB env var for CI/production
- Add PostgreSQL service to Woodpecker CI pipeline
- Create integration tests for migrations (TC-INT-001..002)
- Create integration tests for SQLAlchemyPostRepository (TC-INT-003..009)
- Add unit test for init_db skip behavior (TC-UNIT-901)
- All 176 tests pass, coverage 72.59%

alembic/script.py.mako

@@ -0,0 +1,30 @@
+"""${message}
+
+Revision ID: ${up_revision}
+Revises: ${down_revision | comma,n}
+Create Date: ${create_date}
+
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+
+from alembic import op
+
+${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"}