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/utils/sanitize_test.go

@@ -0,0 +1,71 @@
+package utils
+
+import (
+	"errors"
+	"strings"
+	"testing"
+)
+
+func TestSanitizeMessageText_TrimsWhitespace(t *testing.T) {
+	got, err := SanitizeMessageText("  سلام دوست من  ")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if got != "سلام دوست من" {
+		t.Fatalf("expected trimmed text, got %q", got)
+	}
+}
+
+func TestSanitizeMessageText_EmptyAfterTrim(t *testing.T) {
+	_, err := SanitizeMessageText("     ")
+	if !errors.Is(err, ErrMessageEmpty) {
+		t.Fatalf("expected ErrMessageEmpty, got %v", err)
+	}
+}
+
+func TestSanitizeMessageText_StripsControlChars(t *testing.T) {
+	got, err := SanitizeMessageText("سلام\x00\x07دنیا")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if got != "سلامدنیا" {
+		t.Fatalf("expected control chars stripped, got %q", got)
+	}
+}
+
+func TestSanitizeMessageText_KeepsNewlinesAndTabs(t *testing.T) {
+	got, err := SanitizeMessageText("خط اول\nخط دوم\tبا تب")
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+	if got != "خط اول\nخط دوم\tبا تب" {
+		t.Fatalf("expected newline/tab preserved, got %q", got)
+	}
+}
+
+func TestSanitizeMessageText_InvalidUTF8(t *testing.T) {
+	invalid := string([]byte{0xff, 0xfe, 0xfd})
+	_, err := SanitizeMessageText(invalid)
+	if !errors.Is(err, ErrInvalidMessageEncoding) {
+		t.Fatalf("expected ErrInvalidMessageEncoding, got %v", err)
+	}
+}
+
+func TestSanitizeMessageText_TooLong(t *testing.T) {
+	long := strings.Repeat("a", MaxMessageLength+1)
+	_, err := SanitizeMessageText(long)
+	if !errors.Is(err, ErrMessageTooLong) {
+		t.Fatalf("expected ErrMessageTooLong, got %v", err)
+	}
+}
+
+func TestSanitizeMessageText_ExactlyMaxLength(t *testing.T) {
+	exact := strings.Repeat("a", MaxMessageLength)
+	got, err := SanitizeMessageText(exact)
+	if err != nil {
+		t.Fatalf("unexpected error at exact max length: %v", err)
+	}
+	if len(got) != MaxMessageLength {
+		t.Fatalf("expected length %d, got %d", MaxMessageLength, len(got))
+	}
+}