Add multi-provider LLM support and settings

[mathewab]

This pull request introduces major enhancements to support multiple LLM providers (OpenAI, Anthropic, Google Gemini, Ollama, and Vercel AI Gateway) using a provider-agnostic architecture. It also adds a new persistent app config store, updates environment and configuration handling, and removes the legacy OpenAIAdapter in favor of a unified interface. These changes make the system more flexible, extensible, and easier to configure for different AI backends.

Key changes:

Multi-provider LLM support and configuration

• Added support for multiple LLM providers (OpenAI, Anthropic, Google, Ollama, Gateway) via the Vercel AI SDK, replacing the previous OpenAI-only integration. Environment variables, configuration schemas, and documentation have been updated to reflect this change. (.env.example, README.md, package.json, src/infra/env.ts, [1] [2] [3] [4] [5] [6] [7]

• Introduced a new provider-agnostic LLM adapter interface (LLMAdapter) with support for schema-based JSON outputs, runtime provider selection, and capability introspection. (src/infra/llm/LLMAdapter.ts, src/infra/llm/LLMAdapter.tsR1-R34)

• Removed the legacy OpenAIAdapter and replaced all usage with the new unified LLM adapter approach. (src/infra/OpenAIAdapter.ts, src/infra/OpenAIAdapter.tsL1-L237)

App configuration persistence

• Added a new app_config database table and migration to persist runtime settings (e.g., LLM provider/model) in a key-value format. (src/infra/db/migrations/20260104_add_app_config.cjs, [1]; src/infra/db/schema.sql, [2]

API and domain changes

• Refactored the /config API endpoints to support dynamic LLM provider/model configuration using a new LLMConfigService, with validation and error handling. (src/api/configRoutes.ts, [1]; src/api/index.ts, [2] [3] [4]

• Updated domain error handling to generalize OpenAI errors as LLM errors and expanded audit event types to include LLM provider changes and failures. (src/domain/errors.ts, [1]; src/domain/entities/AuditEntry.ts, [2]