feat(search): retrieval telemetry — log every semantic retrieval

Add retrieval_logs (migration 0068) + services/retrieval_telemetry with a
fire-and-forget record_retrieval(), wired into the MCP search tool
(source=mcp_search) and the REST search route (source=rest_search). Captures
query, effective params, and the per-result score distribution so KB-injection
thresholds can be tuned from data rather than guessed.

Scribe: project 2, milestone 93, task 1032.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Xz4j1H7pjYSjKsEpgcNH5E

src/scribe/mcp/tools/search.py

@@ -7,8 +7,11 @@ working. Differences from fable-mcp:
 """
 from __future__ import annotations
 
+import time
+
 from scribe.mcp._context import current_user_id
-from scribe.services.embeddings import semantic_search_notes
+from scribe.services.embeddings import DEFAULT_SIMILARITY_THRESHOLD, semantic_search_notes
+from scribe.services.retrieval_telemetry import record_retrieval
 
 
 async def search(
@@ -43,10 +46,17 @@ async def search(
     uid = current_user_id()
     limit = max(1, min(limit, 50))
     is_task = {"note": False, "task": True}.get(content_type)  # None => any
+    t0 = time.perf_counter()
     raw = await semantic_search_notes(
         uid, q, limit=limit, is_task=is_task,
         project_id=project_id or None,
     )
+    record_retrieval(
+        user_id=uid, source="mcp_search", query=q,
+        threshold=DEFAULT_SIMILARITY_THRESHOLD, limit=limit,
+        project_id=project_id or None, is_task=is_task, results=raw,
+        duration_ms=(time.perf_counter() - t0) * 1000.0,
+    )
     return {
         "results": [
             {