Pre-Phase-7: per-source LLM routing seam

From the PR description

Summary

• Defer original Phase 7 (local inference / Ollama / vLLM) to post-launch, and land the per-source LLM routing seam now so it's in place before Phase 7 connectors. See decisions.md (2026-05-15) and the renumbered build plan in CLAUDE.md.
• New resolveModelRouting() resolver with document → project → request precedence. Stored prefs are validated against the canonical model set; unknown IDs are rejected and recorded, then the resolver walks down the chain. Conflicts between documents are captured but the first non-null wins (no policy engine yet).
• streamChatWithTools accepts an optional routing context, dispatches with the resolved model, and records the resolution into the existing audit_log.routing_policy_applied jsonb column shipped in Phase 6.
• Main chat path wired to pass the chat's project + the document IDs from docIndex. Today this resolves to the user's requested model unchanged; Phase 7 connectors will populate documents.model_preference at ingest and the same code path will route accordingly without further changes.
• New model_preference (nullable text) columns on projects and documents. No UI yet - by design.

Why now, not in Phase 7

The local-inference adapter is cheap to add later (a fourth file alongside claude.ts/gemini.ts/openai.ts). The routing policy surface is the part that would otherwise get baked into every connector and dispatch site in Phase 7+, and retrofitting it would force a much wider edit later. Landing the seam now keeps Phase 7 connectors free to declare a model_preference at ingest without inventing the surface from scratch.

Test plan

• backend/npx tsc --noEmit clean
• backend/npx vitest run - 73/73 green (8 new routing tests covering precedence, conflicts, unknown-model rejection at doc + project layers, db-error tolerance, and missing-document-ids skip path)
• Manual: with the migration applied, set a documents.model_preference to a known model id on one doc; confirm a chat that includes that doc resolves to the override and writes the policy into audit_log.routing_policy_applied
• Manual: confirm a chat that includes no documents and no project override still works and audit_log.routing_policy_applied records source: "request"

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