files: seek anywhere in a re-encode; it restarts from there

A re-encode's playlist is now the whole film on a fixed 4 s grid. A segment
that isn't on disk and isn't coming soon moves the encode there; runs stop
short of segments already made, and gaps are filled after. Every run is cut
the same way (-ss, -output_ts_offset start+10, frag_discont), so segments
from different runs join. Repackaging keeps its growing playlist.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/play.go

@@ -146,24 +146,48 @@ func (s *Server) playStop(w http.ResponseWriter, r *http.Request) {
 }
 
 // playFile: GET /api/files/play/{id}/{file} — the playlist, the init segment, a segment.
+// A seekable stream's playlist is written whole, up front; a segment of it that isn't on disk
+// yet is waited for, or re-encoded from there if the encode isn't heading that way (a seek).
 func (s *Server) playFile(w http.ResponseWriter, r *http.Request) {
 	st, ok := s.plays.Find(r.PathValue("id"))
 	if !ok {
 		jsonErr(w, 404, "no such stream")
 		return
 	}
-	p, ok := st.File(r.PathValue("file"))
+	name := r.PathValue("file")
+	p, ok := st.File(name)
 	if !ok {
 		jsonErr(w, 404, "not a stream file")
 		return
 	}
+	w.Header().Set("Cache-Control", "no-store")
+	w.Header().Set("X-Content-Type-Options", "nosniff")
+	if st.Seekable {
+		if name == "index.m3u8" {
+			w.Header().Set("Content-Type", "application/vnd.apple.mpegurl")
+			_, _ = w.Write(st.Playlist())
+			return
+		}
+		if i, isSeg := media.SegmentIndex(name); isSeg {
+			sp, err := st.Segment(r.Context(), i)
+			switch {
+			case errors.Is(err, media.ErrNotInFilm):
+				jsonErr(w, 404, err.Error())
+				return
+			case r.Context().Err() != nil:
+				return // the player moved on
+			case err != nil:
+				jsonErr(w, 503, err.Error())
+				return
+			}
+			p = sp
+		}
+	}
 	switch filepath.Ext(p) {
 	case ".m3u8":
 		w.Header().Set("Content-Type", "application/vnd.apple.mpegurl")
 	default:
 		w.Header().Set("Content-Type", "video/mp4")
 	}
-	w.Header().Set("Cache-Control", "no-store")
-	w.Header().Set("X-Content-Type-Options", "nosniff")
 	http.ServeFile(w, r, p)
 }