feat: add alembic migrations, integration tests, and 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/versions/5357028a1574_initial_migration_for_postorm.py

@@ -0,0 +1,48 @@
+"""Initial migration for PostORM.
+
+Revision ID: 5357028a1574
+Revises:
+Create Date: 2026-05-09 20:56:26.292255
+
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+
+from alembic import op
+
+# revision identifiers, used by Alembic.
+revision: str = "5357028a1574"
+down_revision: Union[str, Sequence[str], None] = None
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    op.create_table(
+        "posts",
+        sa.Column("id", sa.String(36), nullable=False),
+        sa.Column("title", sa.String(200), nullable=False),
+        sa.Column("content", sa.Text(), nullable=False),
+        sa.Column("slug", sa.String(200), nullable=False),
+        sa.Column("author_id", sa.String(100), nullable=False),
+        sa.Column("published", sa.Boolean(), nullable=False),
+        sa.Column("tags", sa.JSON(), nullable=False),
+        sa.Column("created_at", sa.DateTime(timezone=True), nullable=False),
+        sa.Column("updated_at", sa.DateTime(timezone=True), nullable=False),
+        sa.PrimaryKeyConstraint("id"),
+        sa.UniqueConstraint("slug"),
+    )
+    op.create_index("ix_posts_author_id", "posts", ["author_id"])
+    op.create_index("ix_posts_published", "posts", ["published"])
+    op.create_index("ix_posts_slug", "posts", ["slug"])
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    op.drop_index("ix_posts_slug", table_name="posts")
+    op.drop_index("ix_posts_published", table_name="posts")
+    op.drop_index("ix_posts_author_id", table_name="posts")
+    op.drop_table("posts")