add: DB-backed settings UI for live-editable tunables

Add a settings page (/settings) and API (/api/settings GET/PUT) to edit
encoding profiles (crf/preset per media type), lp, OMDb key, log retention,
and delete-originals from the browser, persisted to a settings table in
logs.db. config.yaml seeds the store on first run; afterwards the DB is the
source of truth (paths.* and http_addr stay config-only). Each job snapshots
the current settings, so changes apply to the next encode with no restart.

PUT validates ranges (crf 0-63, preset 0-13, lp >=0, retention >=1) before
persisting. No auth, by design for now (isolated network) — see #2.

Closes #1

internal/logger/logger.go

@@ -113,6 +113,18 @@ func (l *Logger) Debug(message, file, extra string) {
 	l.write(LevelDebug, message, file, extra)
 }
 
+// DB exposes the underlying connection so the settings store can share the
+// same logs.db file rather than opening a second database.
+func (l *Logger) DB() *sql.DB { return l.db }
+
+// SetRetention updates the purge horizon at runtime (e.g. from the settings UI).
+// Takes effect on the next purge. Ignored for non-positive values.
+func (l *Logger) SetRetention(days int) {
+	if days > 0 {
+		l.retentionDays = days
+	}
+}
+
 // LogEntry is one row returned by RecentLogs, shaped for the status feed.
 type LogEntry struct {
 	TS      string `json:"ts"`