feat(ai): add server-side provider registry and openai_compatible adapter

[flooryyyy]

Motivation

• Provide a pluggable server-side AI provider layer so the app can support multiple AI backends via configuration.
• Add an OpenAI-compatible adapter to enable using arbitrary OpenAI-like endpoints with configurable baseUrl, apiKey, model, and extra headers.

Description

• Add provider contracts and types in src/app/lib/ai/types.ts (AIProviderType, configs, AIProviderAdapter, chat request/message types).
• Implement a provider registry in src/app/lib/ai/registry.ts with registerAIProvider, resolveAIProvider, and getRegisteredAIProviders for runtime resolution by provider type.
• Implement an openai_compatible adapter in src/app/lib/ai/providers/openai-compatible.ts that normalizes baseUrl, posts to /chat/completions, sets Authorization: Bearer <apiKey>, includes configured model, and merges optional headers.
• Add an AI module entrypoint src/app/lib/ai/index.ts which registers the built-in openai_compatible provider and re-exports types/registry APIs.
• Add unit tests src/app/lib/ai/registry.test.ts validating provider resolution, URL/body construction, and header propagation.

Testing

• Ran npm run test -- src/app/lib/ai/registry.test.ts and the test file passed (2 tests, 2 passed).
• Ran npm run check (ESLint + tsc --noEmit) and it completed successfully.
• Ran full vitest run as part of commit checks and the test suite completed successfully.

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Codex Task