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/routes/search.py

@@ -1,7 +1,14 @@
+import time
+
 from quart import Blueprint, jsonify, request
 
 from scribe.auth import login_required, get_current_user_id
 from scribe.services.embeddings import semantic_search_notes
+from scribe.services.retrieval_telemetry import record_retrieval
+
+# This route searches with a looser floor than the MCP tool default — it powers
+# an interactive feed where loosely-related hits still have value.
+_REST_SEARCH_THRESHOLD = 0.3
 
 search_bp = Blueprint("search", __name__, url_prefix="/api/search")
 
@@ -27,8 +34,15 @@ async def search_route():
     limit = min(request.args.get("limit", 10, type=int), 50)
     is_task = _content_type_to_is_task(content_type)
 
+    t0 = time.perf_counter()
     results = await semantic_search_notes(
-        uid, q, limit=limit, is_task=is_task, threshold=0.3
+        uid, q, limit=limit, is_task=is_task, threshold=_REST_SEARCH_THRESHOLD
+    )
+    record_retrieval(
+        user_id=uid, source="rest_search", query=q,
+        threshold=_REST_SEARCH_THRESHOLD, limit=limit,
+        project_id=None, is_task=is_task, results=results,
+        duration_ms=(time.perf_counter() - t0) * 1000.0,
     )
     return jsonify({
         "results": [