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%

tests/unit/infrastructure/database/test_connection.py

@@ -0,0 +1,18 @@
+import os
+from unittest.mock import patch
+
+import pytest
+
+from app.infrastructure.database.connection import init_db
+
+
+class TestInitDB:
+    @pytest.mark.asyncio
+    async def test_init_db_skipped_when_skip_env_set(self) -> None:
+        with patch.dict(os.environ, {"SKIP_INIT_DB": "1"}):
+            await init_db()
+
+    @pytest.mark.asyncio
+    async def test_init_db_runs_when_no_env(self) -> None:
+        with patch.dict(os.environ, {}, clear=True):
+            await init_db()