feat: add domain exception classes and wire into throw sites

[ksandell]

Summary

• Add 4 missing exception classes documented in the API Reference: CacheException, ConfigException, DatabaseException, MiddlewareException — all extending SpinException
• Wire CacheException into src/Cache/Adapters/Apcu.php and Redis.php (replaces \RuntimeException)
• Wire ConfigException into src/Core/Config.php (replaces SpinException, adds $previous chaining on JSON parse errors)
• Wire DatabaseException into src/Database/PdoConnection.php (rawQuery/rawExec) and src/Core/ConnectionManager.php (password-sanitization logic preserved — no $previous on sanitized path)
• Add 28 new PHPUnit tests (tests/Exceptions/) — 7 per exception class
• Update PdoConnectionInterface @throws docblocks
• Update changelog.md, composer.json, package.json, VERSION to 0.0.40

Exception hierarchy

\Exception
  └── Spin\Exceptions\SpinException  (existing)
        ├── Spin\Exceptions\CacheException
        ├── Spin\Exceptions\ConfigException
        ├── Spin\Exceptions\DatabaseException
        └── Spin\Exceptions\MiddlewareException

Test plan

• ./vendor/bin/phpunit tests/Exceptions/ — 28 tests, all PASS
• ./vendor/bin/phpunit tests/Cache/ — no regressions (APCu/Redis skipped if unavailable)
• ./vendor/bin/phpunit tests/Database/ — 2 new exception-type tests PASS with live MySQL, SKIP without
• Full suite: ./vendor/bin/phpunit — no new failures