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/db/postgres.rs

@@ -21,8 +21,8 @@ use crate::db::{
 };
 use crate::error::{DatabaseError, WorkspaceError};
 use crate::history::{
-    ConversationMessage, ConversationSummary, JobEventRecord, LlmCallRecord, SandboxJobRecord,
-    SandboxJobSummary, SettingRow, Store,
+    AgentJobRecord, AgentJobSummary, ConversationMessage, ConversationSummary, JobEventRecord,
+    LlmCallRecord, SandboxJobRecord, SandboxJobSummary, SettingRow, Store,
 };
 use crate::workspace::{
     MemoryChunk, MemoryDocument, Repository, SearchConfig, SearchResult, WorkspaceEntry,
@@ -215,6 +215,14 @@ impl JobStore for PgBackend {
         self.store.get_stuck_jobs().await
     }
 
+    async fn list_agent_jobs(&self) -> Result<Vec<AgentJobRecord>, DatabaseError> {
+        self.store.list_agent_jobs().await
+    }
+
+    async fn agent_job_summary(&self) -> Result<AgentJobSummary, DatabaseError> {
+        self.store.agent_job_summary().await
+    }
+
     async fn save_action(&self, job_id: Uuid, action: &ActionRecord) -> Result<(), DatabaseError> {
         self.store.save_action(job_id, action).await
     }