feat(data): add subtasks table, repository and prompt integration

Per-task checklist backend: subtasks table with CASCADE delete,
SubtaskEntity + SubtaskRepository (connection-per-op, async), DI
registration in App and Worker, TaskRunner composes a '## Sub-Tasks'
markdown block into the Claude prompt when subtasks exist.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

schema/schema.sql

@@ -85,6 +85,16 @@ CREATE TABLE IF NOT EXISTS task_runs (
 
 CREATE INDEX IF NOT EXISTS idx_task_runs_task_id ON task_runs(task_id);
 
+CREATE TABLE IF NOT EXISTS subtasks (
+    id         TEXT PRIMARY KEY,
+    task_id    TEXT NOT NULL REFERENCES tasks(id) ON DELETE CASCADE,
+    title      TEXT NOT NULL,
+    completed  INTEGER NOT NULL DEFAULT 0,
+    order_num  INTEGER NOT NULL,
+    created_at TIMESTAMP NOT NULL
+);
+CREATE INDEX IF NOT EXISTS idx_subtasks_task_id ON subtasks(task_id);
+
 -- Seed: minimal tag set (ignored if already present)
 INSERT OR IGNORE INTO tags (name) VALUES ('agent');
 INSERT OR IGNORE INTO tags (name) VALUES ('manual');