perf: resolve hops from in-memory prefix map instead of N+1 DB queries (#577)

## Summary

Replace N+1 per-hop DB queries in `handleResolveHops` with O(1) lookups
against the in-memory prefix map that already exists in the packet
store.

## Problem

Each hop in the `resolve-hops` API triggered a separate `SELECT ... LIKE
?` query against the nodes table. With 10 hops, that's 10 DB round-trips
— unnecessary when `getCachedNodesAndPM()` already maintains an
in-memory prefix map that can resolve hops instantly.

## Changes

- **routes.go**: Replace the per-hop DB query loop with `pm.m[hopLower]`
lookups from the prefix map. Convert `nodeInfo` → `HopCandidate` inline.
Remove unused `rows`/`sql.Scan` code.
- **store.go**: Add `InvalidateNodeCache()` method to force prefix map
rebuild (needed by tests that insert nodes after store initialization).
- **routes_test.go**: Give `TestResolveHopsAmbiguous` a proper store so
hops resolve via the prefix map.
- **resolve_context_test.go**: Call `InvalidateNodeCache()` after
inserting test nodes. Fix confidence assertion — with GPS candidates and
no affinity context, `resolveWithContext` correctly returns
`gps_preference` (previously masked because the prefix map didn't have
the test nodes).

## Complexity

O(1) per hop lookup via hash map vs O(n) DB scan per hop. No hot-path
impact — this endpoint is called on-demand, not in a render loop.

Fixes #369

---------

Co-authored-by: you <you@example.com>

cmd/server/resolve_context_test.go

@@ -166,6 +166,7 @@ func TestResolveHopsAPI_UniquePrefix(t *testing.T) {
 	// Insert a unique node
 	srv.db.conn.Exec("INSERT OR IGNORE INTO nodes (public_key, name, lat, lon) VALUES (?, ?, ?, ?)",
 		"ff11223344", "UniqueNode", 37.0, -122.0)
+	srv.store.InvalidateNodeCache()
 
 	req := httptest.NewRequest("GET", "/api/resolve-hops?hops=ff11223344", nil)
 	rr := httptest.NewRecorder()
@@ -192,6 +193,7 @@ func TestResolveHopsAPI_AmbiguousNoContext(t *testing.T) {
 		"ee1aaaaaaa", "Node-E1", 37.0, -122.0)
 	srv.db.conn.Exec("INSERT OR IGNORE INTO nodes (public_key, name, lat, lon) VALUES (?, ?, ?, ?)",
 		"ee1bbbbbbb", "Node-E2", 38.0, -121.0)
+	srv.store.InvalidateNodeCache()
 
 	req := httptest.NewRequest("GET", "/api/resolve-hops?hops=ee1", nil)
 	rr := httptest.NewRecorder()
@@ -204,8 +206,10 @@ func TestResolveHopsAPI_AmbiguousNoContext(t *testing.T) {
 	if hr == nil {
 		t.Fatal("expected hop in resolved map")
 	}
-	if hr.Confidence != "ambiguous" {
-		t.Fatalf("expected ambiguous, got %s", hr.Confidence)
+	// With both candidates having GPS and no affinity context, the resolver
+	// picks the GPS-preferred candidate → confidence is "gps_preference".
+	if hr.Confidence != "gps_preference" {
+		t.Fatalf("expected gps_preference, got %s", hr.Confidence)
 	}
 	if len(hr.Candidates) != 2 {
 		t.Fatalf("expected 2 candidates, got %d", len(hr.Candidates))

cmd/server/routes.go

@@ -1419,24 +1419,25 @@ func (s *Server) handleResolveHops(w http.ResponseWriter, r *http.Request) {
 			continue
 		}
 		hopLower := strings.ToLower(hop)
-		rows, err := s.db.conn.Query("SELECT public_key, name, lat, lon FROM nodes WHERE LOWER(public_key) LIKE ?", hopLower+"%")
-		if err != nil {
-			resolved[hop] = &HopResolution{Name: nil, Candidates: []HopCandidate{}, Conflicts: []interface{}{}, Confidence: "ambiguous"}
-			continue
-		}
 
+		// Resolve candidates from the in-memory prefix map instead of
+		// issuing per-hop DB queries (fixes N+1 pattern, see #369).
 		var candidates []HopCandidate
-		for rows.Next() {
-			var pk string
-			var name sql.NullString
-			var lat, lon sql.NullFloat64
-			rows.Scan(&pk, &name, &lat, &lon)
-			candidates = append(candidates, HopCandidate{
-				Name: nullStr(name), Pubkey: pk,
-				Lat: nullFloat(lat), Lon: nullFloat(lon),
-			})
+		if pm != nil {
+			if matched, ok := pm.m[hopLower]; ok {
+				for _, ni := range matched {
+					c := HopCandidate{Pubkey: ni.PublicKey}
+					if ni.Name != "" {
+						c.Name = ni.Name
+					}
+					if ni.HasGPS {
+						c.Lat = ni.Lat
+						c.Lon = ni.Lon
+					}
+					candidates = append(candidates, c)
+				}
+			}
 		}
-		rows.Close()
 
 		if len(candidates) == 0 {
 			resolved[hop] = &HopResolution{Name: nil, Candidates: []HopCandidate{}, Conflicts: []interface{}{}, Confidence: "no_match"}

cmd/server/routes_test.go

@@ -1170,6 +1170,11 @@ func TestResolveHopsAmbiguous(t *testing.T) {
 	cfg := &Config{Port: 3000}
 	hub := NewHub()
 	srv := NewServer(db, cfg, hub)
+	store := NewPacketStore(db, nil)
+	if err := store.Load(); err != nil {
+		t.Fatalf("store.Load failed: %v", err)
+	}
+	srv.store = store
 	router := mux.NewRouter()
 	srv.RegisterRoutes(router)
 

cmd/server/store.go

@@ -3726,6 +3726,15 @@ func (s *PacketStore) getCachedNodesAndPM() ([]nodeInfo, *prefixMap) {
 	return nodes, pm
 }
 
+// InvalidateNodeCache forces the next getCachedNodesAndPM call to rebuild.
+func (s *PacketStore) InvalidateNodeCache() {
+	s.cacheMu.Lock()
+	s.nodeCache = nil
+	s.nodePM = nil
+	s.nodeCacheTime = time.Time{}
+	s.cacheMu.Unlock()
+}
+
 func (pm *prefixMap) resolve(hop string) *nodeInfo {
 	h := strings.ToLower(hop)
 	candidates := pm.m[h]