Add manual scheduled task entry

Lets users log tasks the Linux agent can't discover on its own (e.g.
Docker-based backup jobs) directly in the UI. Tasks now carry an
origin ('agent' | 'manual') so the agent's report-sync logic only
ever creates/updates/stale-marks agent-sourced rows, leaving manual
entries untouched; the API rejects edits/deletes of agent-sourced
tasks to keep that boundary enforced server-side too.

server/drizzle/0001_amazing_greymalkin.sql

@@ -0,0 +1 @@
+ALTER TABLE `scheduled_tasks` ADD `origin` text DEFAULT 'agent' NOT NULL;