Add stats, profiles, avatars and palaute

Step 6. The surfaces around the review loop.

- Nine leaderboards, ordered and limited in SQL, each with a deterministic
  tie-break so a tied board doesn't reshuffle between reloads. Min 3 reviews
  to qualify, for reviewer boards too
- Profiles show counts and history-wide averages and the member's songs,
  never a list of their reviews — per-song opinion stays gated
- Avatars: 5MB in, 256px JPEG out, ffmpeg's re-encode being the validation.
  No upload still means initials, and avatars are public
- Changing your own password requires the current one and drops your other
  sessions
- Palaute: free text plus the page you were on, carried in a footer link, and
  the user agent from the header. Reporters see their own; the admin resolves
  them with a timestamp rather than a status enum
- Admin gained the song list with delete, the reports page, and an open-report
  count on the dashboard

Two theme fixes the screenshots caught: leaderboard ranks need a CSS counter
because display:grid suppresses list markers, and count-based boards were
printing 3.0 where they mean 3.

media.go

@@ -176,6 +176,17 @@ func downloadYouTube(ctx context.Context, url, outTemplate string) (string, erro
 	return "", nil
 }
 
+// toAvatarJPEG normalises any image ffmpeg understands into a 256px square JPEG. The re-encode is
+// the validation and the size cap in one — webp and avif included, which stdlib image cannot read.
+func toAvatarJPEG(ctx context.Context, in, out string) error {
+	ctx, cancel := context.WithTimeout(ctx, 30*time.Second)
+	defer cancel()
+
+	return exec.CommandContext(ctx, "ffmpeg", "-nostdin", "-y", "-i", in,
+		"-vf", "scale=256:256:force_original_aspect_ratio=increase,crop=256:256",
+		"-frames:v", "1", "-q:v", "3", out).Run()
+}
+
 // convertToOpus is also the validation: if ffmpeg produced an Opus stream, the upload was audio.
 // No container sniffing, no magic-byte library. Returns the stderr tail on failure, which is worth
 // showing — "Invalid data found when processing input" beats "submission failed".