feat: API hardening (Pydantic schemas, pagination, Alembic, OpenAPI)

[Kwickos]

Phase 2 — hardens the REST API.

Added

• Typed Pydantic response models (app/schemas/api.py) + generic Page pagination envelope.
• limit/offset pagination on /matches, /games/{id}/events, /games/{id}/frames with total counts.
• Async Alembic migrations with the initial schema (alembic/) — production source of truth (dev still uses create_all).
• OpenAPI tags + richer API metadata (description, contact, license).
• API endpoint tests (tests/test_api.py) + isolated test DB (tests/conftest.py).

Verified

• ruff check/format clean · pytest 8/8 · alembic upgrade head/downgrade base OK (7 tables).