v0.10: /v1/rag endpoint for Router <-> Fabric platform integration

[JasonDD]

Summary

Adds a new POST /v1/rag endpoint so Router can delegate its embedded RAG
pre-stage to Fabric. The endpoint returns ranked chunks (smaller and
content-addressed, unlike memos) with caller-configurable ranking weights.

• New table fabric.chunks (id, source_path, source_range, content,
  content_sha256, metadata jsonb, embedding vector(768), tsv stored)
• New POST /v1/rag with weights {cosine, tsvector, recency}, filters
  {source_paths, type, min_score}, max_chunk_tokens
• New POST /v1/chunks companion upsert endpoint for the (still-separate)
  chunk-ingestion workstream
• Empty-list clean response when chunks table is empty, so the
  integration can be wired and smoke-tested today
• Bumps version to 0.10.0

Companion PR (Router side): Kronaxis/kronaxis-router#1

Test plan

• go build ./... and go vet ./... pass locally
• Deployed to DL580 (192.168.50.129:8201), restarted fabric.service
• /v1/health reports version 0.10.0
• /v1/rag returns {"chunks":[]} cleanly with an empty chunk table
• After seeding two chunks via /v1/chunks, /v1/rag ranks the
  classifier chunk at 0.79 cosine for a router-classifier query
• Router end-to-end smoke test (see companion PR) confirms the
  chat completion is augmented with the retrieved chunks

Out of scope

• Chunk ingestion pipeline (separate workstream)
• Marketing site updates (parallel agent)
• Performance benchmarking, larger test surface

DO NOT merge yet -- operator review.