Refactor: split community.py into focused modules (1976 lines)

[neuromechanist]

Summary

src/api/routers/community.py is the single largest file in the codebase at 1,976 lines. This violates the single responsibility principle and makes it difficult to reason about, test, and modify.

Current Structure (problematic)

The file mixes 4 distinct concerns:

• Pydantic model definitions (ChatRequest, ChatMessage, PageContext, etc.)
• Router factory logic for generating per-community endpoints
• Knowledge search and retrieval integration
• Metrics logging and cost tracking

Proposed Split

Module	Contents	Approx Lines
models.py	Pydantic request/response models	~150 lines
router_factory.py	create_community_router() factory	~400 lines
endpoints/ask.py	/ask endpoint implementation	~200 lines
endpoints/chat.py	/chat endpoint (SSE streaming)	~300 lines
endpoints/sessions.py	Session management endpoints	~150 lines

Acceptance Criteria

• No file exceeds 600 lines after split
• All existing tests continue to pass without modification
• Import paths updated in dependent modules (main.py, test files)
• Public API behavior is unchanged (API contract preserved)