Add navigation & usage metrics for end-user experience visibility

Instrument DB query timing (11 operations across 3 repositories),
streaming lifecycle (TTFB, duration, feature count), cache operation
latency, listing detail step breakdown, and frontend page load /
time-to-first-listing / stream download / detail load metrics.

Adds 16 new OTel instruments, extends the perf ingestion endpoint
with 4 new frontend metrics, and adds ~20 Grafana dashboard panels
across 4 new rows (DB Query Performance, Streaming Performance,
Listing Detail Breakdown, Cache Performance, Frontend Navigation).

repositories/listing_repository.py

@@ -1,6 +1,8 @@
 from datetime import datetime, timedelta
 import logging
+import time
 from typing import Any, Generator
+from api.metrics import record_db_query
 from data_access import Listing
 from models.listing import (
     BuyListing,
@@ -55,8 +57,10 @@ class ListingRepository:
         if limit:
             query = query.limit(limit)
 
+        t0 = time.monotonic()
         with Session(self.engine) as session:
             rows = list(session.exec(query).all())
+        record_db_query("get_listings", model.__tablename__, time.monotonic() - t0, len(rows))
         logging.debug(f"Found {len(rows)} listings")
         return rows
 
@@ -110,8 +114,11 @@ class ListingRepository:
         query = sa_select(func.count(model.id))
         query = self._apply_query_filters(query, model, query_parameters)
 
+        t0 = time.monotonic()
         with Session(self.engine) as session:
-            return session.execute(query).scalar() or 0
+            result = session.execute(query).scalar() or 0
+        record_db_query("count_listings", model.__tablename__, time.monotonic() - t0, result)
+        return result
 
     def stream_listings_optimized(
         self,
@@ -157,8 +164,10 @@ class ListingRepository:
                 batch_limit = min(page_size, limit - total_yielded)
             query = query.order_by(model.id).limit(batch_limit)
 
+            t0 = time.monotonic()
             with Session(self.engine) as session:
                 results = session.execute(query).fetchall()
+            record_db_query("stream_listings_page", model.__tablename__, time.monotonic() - t0, len(results))
 
             if not results:
                 break
@@ -364,6 +373,7 @@ class ListingRepository:
 
         Checks both RentListing and BuyListing tables and returns the latest.
         """
+        t0 = time.monotonic()
         with Session(self.engine) as session:
             rent_max = session.execute(
                 sa_select(func.max(RentListing.last_seen))
@@ -371,6 +381,7 @@ class ListingRepository:
             buy_max = session.execute(
                 sa_select(func.max(BuyListing.last_seen))
             ).scalar()
+        record_db_query("get_last_updated", "rent", time.monotonic() - t0)
 
         candidates = [t for t in (rent_max, buy_max) if t is not None]
         return max(candidates) if candidates else None
@@ -385,9 +396,12 @@ class ListingRepository:
         filtering against API results.
         """
         model = RentListing if listing_type == ListingType.RENT else BuyListing
+        t0 = time.monotonic()
         with Session(self.engine) as session:
             result = session.execute(sa_select(model.id))
-            return {row[0] for row in result.fetchall()}
+            ids = {row[0] for row in result.fetchall()}
+        record_db_query("get_listing_ids", model.__tablename__, time.monotonic() - t0, len(ids))
+        return ids
 
     async def mark_seen(self, listing_id: int, listing_type: ListingType = ListingType.RENT) -> None:
         query_params = QueryParameters(listing_type=listing_type)