feat(web): DB-backed Jobs tab + scheduler-dispatched local jobs (#436)

* feat(web): DB-backed Jobs tab, scheduler-dispatched local jobs, remove active-jobs-bar

- Remove active-jobs-bar UI element (HTML, CSS, JS polling)
- Move job handlers from server.rs to handlers/jobs.rs
- Remove user_id scoping (single-user gateway)
- Add list_agent_jobs() and agent_job_summary() to Database trait
  (both postgres and libsql backends) for non-sandbox job visibility
- Wire SchedulerSlot into CreateJobTool so execute_local dispatches
  via scheduler (persists to DB + spawns worker) instead of creating
  phantom ContextManager-only jobs
- Update /status and /list slash commands to read from DB for
  consistency with Jobs tab
- Fix worker mark_completed: skip if already terminal or stuck
- Add agent job cancel via DB update in both web handler and slash cmd
- Add Stuck → Completed guard with tracing in worker completion path

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

* fix: address PR review comments

- Log warning when get_context fails in worker completion path
- Extract duplicated status-counting logic into AgentJobSummary::add_count()
  helper, used by both postgres and libsql backends

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

---------

Co-authored-by: Claude Sonnet 4.6 <[email protected]>
Co-authored-by: Nick Pismenkov <[email protected]>

src/agent/worker.rs

@@ -158,6 +158,37 @@ Report when the job is complete or if you encounter issues you cannot resolve."#
         match result {
             Ok(Ok(())) => {
                 tracing::info!("Worker for job {} completed successfully", self.job_id);
+                // Only mark completed if still in an active, non-stuck state.
+                // The execution_loop may have already called mark_completed or
+                // mark_stuck (e.g. "plan completed but work remains").
+                let current_state = self
+                    .context_manager()
+                    .get_context(self.job_id)
+                    .await
+                    .map(|ctx| ctx.state);
+                match current_state {
+                    Ok(state) if state.is_terminal() => {
+                        // Already in a terminal state (e.g. execution_loop
+                        // called mark_completed itself).
+                    }
+                    Ok(JobState::Stuck) => {
+                        // execution_loop marked this as stuck (e.g. "plan
+                        // completed but work remains"); leave for self-repair.
+                        tracing::info!(
+                            "Job {} returned Ok but is Stuck — leaving for self-repair",
+                            self.job_id
+                        );
+                    }
+                    Ok(_) => {
+                        self.mark_completed().await?;
+                    }
+                    Err(e) => {
+                        tracing::warn!(
+                            job_id = %self.job_id,
+                            "Failed to get job context, cannot mark as completed: {}", e
+                        );
+                    }
+                }
             }
             Ok(Err(e)) => {
                 tracing::error!("Worker for job {} failed: {}", self.job_id, e);