feat: implement full-stack TaskPilot scheduler

backend/migrations/0001_create_tasks.sql

@@ -0,0 +1,28 @@
+CREATE TABLE IF NOT EXISTS tasks (
+    id TEXT PRIMARY KEY NOT NULL,
+    name TEXT NOT NULL,
+    cron TEXT NOT NULL,
+    timezone TEXT NOT NULL,
+    enabled INTEGER NOT NULL CHECK (enabled IN (0, 1)),
+    timeout_secs INTEGER NOT NULL CHECK (timeout_secs > 0),
+    concurrency_policy TEXT NOT NULL DEFAULT 'skip'
+        CHECK (concurrency_policy = 'skip'),
+    retry_max_attempts INTEGER NOT NULL DEFAULT 0
+        CHECK (retry_max_attempts >= 0),
+    retry_delay_secs INTEGER NOT NULL DEFAULT 0
+        CHECK (retry_delay_secs >= 0),
+    action_type TEXT NOT NULL
+        CHECK (action_type IN ('script', 'http', 'process')),
+    action_json TEXT NOT NULL,
+    created_at TEXT NOT NULL,
+    updated_at TEXT NOT NULL
+);
+
+CREATE INDEX IF NOT EXISTS idx_tasks_updated_at
+    ON tasks(updated_at DESC);
+
+CREATE INDEX IF NOT EXISTS idx_tasks_enabled
+    ON tasks(enabled);
+
+CREATE INDEX IF NOT EXISTS idx_tasks_action_type
+    ON tasks(action_type);