script-mode task execution — database-backed tasks with direct script runner

Tasks now live in the database (mode: script or agentic). Script-mode tasks
bypass the agent entirely — the implementation is materialized to a temp file
and executed directly, with stdout/stderr streamed to the UI via WebSocket.

Includes convert-to-mp3 as the first native script task.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

seed/tasks/convert-to-mp3/TASK.md

@@ -0,0 +1,35 @@
+---
+name: Convert To MP3
+description: Convert audio files to MP3 320kbps, preserving metadata.
+version: 1
+mode: script
+language: bash
+triggers:
+  - type: file
+    extensions:
+      - flac
+      - wav
+      - ogg
+      - wma
+      - aac
+      - m4a
+      - opus
+      - aiff
+      - aif
+      - ape
+      - wv
+      - alac
+      - dsf
+      - dff
+  - type: directory
+inputs:
+  file_path:
+    type: string
+    description: Path to an audio file or directory to convert.
+args: [file_path]
+---
+
+# Convert To MP3
+
+Convert audio files to MP3 320kbps using ffmpeg, preserving metadata.
+Supports single file conversion and batch directory conversion.