add: live encode progress tracking and read-only status web UI

Stream ffmpeg -progress during the video encode into an in-memory tracker
(internal/status) so the long-running step is no longer a black box: percent,
fps, speed, and ETA are derived from the source duration and updated ~1/s.
Progress prints to stdout only (no logs.db row) to avoid burying real events.

Expose it over HTTP (internal/server, default :8080, set http_addr to "" to
disable): GET /status returns the live snapshot, the pending input queue, and
recent non-debug events; GET / serves an embedded dashboard that polls /status
every second. The server shuts down on the same SIGINT/SIGTERM context as the
watcher.

internal/watcher/watcher.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"sort"
 	"time"
 
 	"videnc-vibe/pkg/types"
@@ -28,6 +29,19 @@ var inputExtensions = []string{
 	"webm", "wmv", "flv",
 }
 
+// InputFiles returns the source files currently in dir that the watcher would
+// consider, sorted. The status server uses this to report the pending queue, so
+// it stays in sync with inputExtensions.
+func InputFiles(dir string) []string {
+	var files []string
+	for _, ext := range inputExtensions {
+		matches, _ := filepath.Glob(filepath.Join(dir, "*."+ext))
+		files = append(files, matches...)
+	}
+	sort.Strings(files)
+	return files
+}
+
 // fileStat is the (mtime, size) pair used to decide whether a file has settled
 // between two consecutive ticks.
 type fileStat struct {