Add production hardening: monitoring, backups, caching, and high-traffic resilience

- Recovery middleware (no more crash-on-panic), request metrics/monitoring
  store + admin API, in-memory TTL caching for read-heavy option/question/
  category lookups, gzip + rate-limiting middleware, sanitized chat message
  encoding, and pg_dump-based database backups with an admin API.
- Fix N+1 queries in the discover feed, candidate detail, and match list
  endpoints by batching selection/photo/user/last-message lookups into
  single queries instead of one-per-item loops.
- Tune DB connection pooling and add HTTP server timeouts + graceful
  shutdown so the API degrades gracefully instead of falling over under load.
- Fix a null (vs empty array) JSON response for candidates/matches without
  photos or interests, which crashed the web app's swipe card.

internal/cache/ttl_cache_test.go

@@ -0,0 +1,62 @@
+package cache
+
+import (
+	"testing"
+	"time"
+)
+
+func TestTTLCache_SetGet(t *testing.T) {
+	c := New[string, int](time.Minute)
+	c.Set("a", 42)
+
+	v, ok := c.Get("a")
+	if !ok || v != 42 {
+		t.Fatalf("expected (42, true), got (%d, %v)", v, ok)
+	}
+}
+
+func TestTTLCache_MissingKey(t *testing.T) {
+	c := New[string, int](time.Minute)
+	if _, ok := c.Get("missing"); ok {
+		t.Fatal("expected miss for key that was never set")
+	}
+}
+
+func TestTTLCache_Expiry(t *testing.T) {
+	c := New[string, int](10 * time.Millisecond)
+	c.Set("a", 1)
+
+	if _, ok := c.Get("a"); !ok {
+		t.Fatal("expected hit immediately after Set")
+	}
+
+	time.Sleep(30 * time.Millisecond)
+
+	if _, ok := c.Get("a"); ok {
+		t.Fatal("expected miss after TTL expired")
+	}
+}
+
+func TestTTLCache_Delete(t *testing.T) {
+	c := New[string, int](time.Minute)
+	c.Set("a", 1)
+	c.Delete("a")
+
+	if _, ok := c.Get("a"); ok {
+		t.Fatal("expected miss after Delete")
+	}
+}
+
+func TestTTLCache_Clear(t *testing.T) {
+	c := New[string, int](time.Minute)
+	c.Set("a", 1)
+	c.Set("b", 2)
+	c.Clear()
+
+	if _, ok := c.Get("a"); ok {
+		t.Fatal("expected miss for 'a' after Clear")
+	}
+	if _, ok := c.Get("b"); ok {
+		t.Fatal("expected miss for 'b' after Clear")
+	}
+}