Patch Source Type

Update a source's type (Phase 17: user override from dropdown).

Sets source_type and resolved_kind so Tier 2 uses this type when running "Process with AI". Storage-gated: editing the type is file metadata; the AI processing it feeds stays behind ai_layer.

NOTE (JEEVY-1274): this is one of THREE writers of resolved_kind — the only one that writes it from an explicit HUMAN choice rather than the tier-1 heuristic classifier (context_sync.py, deliverable_service.py). resolved_kind cannot carry that distinction on its own (same value vocabulary as the heuristic writers) — this endpoint also stamps the dedicated sources.source_type_origin = 'user' column so semantic_classify can respect this choice as AUTHORITATIVE rather than argue with it in prompt text.