test: add GraphQL, SQLite persistence, and API validation integration tests

Add three new test suites that exercise the server against real storage
with no mocks:

- crates/axon-server/tests/graphql_contract.rs: 20 Rust integration tests
  covering every GraphQL endpoint via axum_test::TestServer backed by
  SQLite in-memory. Covers introspection, CRUD mutations, list/pagination,
  VERSION_CONFLICT and SCHEMA_VALIDATION error codes, delete semantics,
  and the playground endpoint.

- ui/tests/e2e/persistence.spec.ts: Playwright tests that query the SQLite
  database file directly via the sqlite3 CLI to prove disk durability after
  each API write.

- ui/tests/e2e/validation.spec.ts: Playwright tests covering 422/409/404
  error paths and schema constraint validation (enum, range, string length,
  additionalProperties, multi-violation reporting).

- ui/playwright.e2e.sqlite.config.ts: Playwright config that starts axon
  serve with SQLite storage for the persistence suite.

- ui/package.json: added test:e2e:sqlite script.

Verification: cargo test -p axon-server --test graphql_contract,
cargo clippy -p axon-server --tests -- -D warnings

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: easel