feat: add MCP server with sql_query, web_fetch, web_search tools

[dimslaev]

Adds a Model Context Protocol (MCP) server that exposes three tools for agents to query the database, fetch web pages, and search the web.

Key changes:

• New MCP server (app/mcp/server.py) with bearer-token authentication gated by MCP_TOKEN setting
• Three tools in app/mcp/tools.py:
  • sql_query: read-only database access via agentique_ro role, with statement planning to block writes and access to the user table, 1000-row result cap, and JSON serialization for timestamps/numerics
  • web_fetch: extracts readable text from a URL with proxy retry on 403/paywall/login blocks
  • web_search: Tavily web search with result ranking and optional domain filtering
• New agentique_ro database role (passwordless on socket, password-protected over TCP) with SELECT-only grants and statement/lock timeouts
• Settings for MCP token, read-only database user/password, and computed DSN with read-only transaction and 10s statement timeout
• Mounted at /mcp/ in the main FastAPI app with CORS headers for SSE streaming
• Vite proxy and Caddy reverse proxy configuration for /mcp endpoint
• .mcp.json client config for agent integration
• Comprehensive tests for query guards, row capping, type coercion, and tool forwarding
• Workflow dispatch for SQL changed from read/write modes to write-only (reads now use MCP tool)

Implementation details:

• Statement planning via EXPLAIN (FORMAT JSON) walks the entire plan tree to catch blocked tables in subqueries, CTEs, and joins
• Prepared statements ensure only one statement executes, preventing semicolon-based injection
• _json_safe fallback handles datetime, Decimal, UUID, and pgvector types for JSON serialization
• Read-only transaction and role grants provide defense-in-depth against accidental writes

https://claude.ai/code/session_014AwCiyN5ksZ5QsHqXq8RQw