docs: describe SQLite logging (logs.db), drop stale log-file docs

Logging moved to a logs.db SQLite store, but MANUAL.md, MANUAL.html and
SPEC.md still documented info_*.log / error_*.log / structured.json and a
midnight-rollover limitation that no longer exist. Replace those sections with
the logs.db reality: levels (info/error to db+stdout/stderr, debug db-only),
progress-to-stdout-only, retention via log_retention_days, and that recent
events are viewable in the dashboard / GET /status.

Closes #6

SPEC.md

@@ -99,9 +99,10 @@ Regex patterns:
 
 ## Logging
 
-- info.log: Human-readable info level
-- error.log: Human-readable error level
-- structured.json: JSON structured logs
+- All logs are stored in a SQLite database `logs.db` in the working directory.
+- Levels: `debug`, `info`, `error`. `info`/`error` also print to stdout/stderr; `debug` (ffprobe/ffmpeg/API dumps) is database-only. Live encode progress prints to stdout only (not stored).
+- Retention: rows older than `log_retention_days` (default 7) are purged on startup/shutdown; editable from the settings UI.
+- Recent non-debug events are viewable in the web dashboard and via `GET /status`.
 
 ## Polling
 - Scan input folder every 10-30 seconds for `.mkv` files