Add in-memory result cache with 1-hour TTL

DB is now queried at most once per hour per user; subsequent requests
within the window are served from memory. TTL matches the existing
Cache-Control header. Restart clears the cache immediately.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

main.go

@@ -21,6 +21,7 @@ import (
 	"os"
 	"os/signal"
 	"strings"
+	"sync"
 	"syscall"
 	"time"
 
@@ -83,12 +84,43 @@ func getEnv(key, fallback string) string {
 	return fallback
 }
 
+// ---- cache ----------------------------------------------------------------
+
+const cacheTTL = time.Hour
+
+type cacheEntry struct {
+	data    []dayCount
+	expires time.Time
+}
+
+type resultCache struct {
+	mu      sync.Mutex
+	entries map[string]cacheEntry
+}
+
+func (c *resultCache) get(username string) ([]dayCount, bool) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	e, ok := c.entries[username]
+	if !ok || time.Now().After(e.expires) {
+		return nil, false
+	}
+	return e.data, true
+}
+
+func (c *resultCache) set(username string, data []dayCount) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+	c.entries[username] = cacheEntry{data: data, expires: time.Now().Add(cacheTTL)}
+}
+
 // ---- handler --------------------------------------------------------------
 
 type heatmapHandler struct {
-	db  *sql.DB
-	cfg *config
-	log *slog.Logger
+	db    *sql.DB
+	cfg   *config
+	log   *slog.Logger
+	cache *resultCache
 }
 
 type dayCount struct {
@@ -118,17 +150,22 @@ func (h *heatmapHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	ctx, cancel := context.WithTimeout(r.Context(), 5*time.Second)
-	defer cancel()
+	counts, ok := h.cache.get(username)
+	if !ok {
+		ctx, cancel := context.WithTimeout(r.Context(), 5*time.Second)
+		defer cancel()
 
-	counts, err := h.queryHeatmap(ctx, username)
-	if err != nil {
-		h.log.Error("heatmap query failed", "user", username, "err", err)
-		http.Error(w, "internal error", http.StatusInternalServerError)
-		return
-	}
-	if counts == nil {
-		counts = []dayCount{}
+		var err error
+		counts, err = h.queryHeatmap(ctx, username)
+		if err != nil {
+			h.log.Error("heatmap query failed", "user", username, "err", err)
+			http.Error(w, "internal error", http.StatusInternalServerError)
+			return
+		}
+		if counts == nil {
+			counts = []dayCount{}
+		}
+		h.cache.set(username, counts)
 	}
 
 	w.Header().Set("Content-Type", "application/json")
@@ -216,7 +253,12 @@ func main() {
 		os.Exit(1)
 	}
 
-	h := &heatmapHandler{db: db, cfg: cfg, log: logger}
+	h := &heatmapHandler{
+		db:    db,
+		cfg:   cfg,
+		log:   logger,
+		cache: &resultCache{entries: make(map[string]cacheEntry)},
+	}
 
 	mux := http.NewServeMux()
 	mux.Handle("/heatmap/", h)