feat: add cursor-based pagination to /jobs endpoint

Add a cursor query parameter to GET /jobs and return a next_cursor
field in the response envelope. Existing clients using only limit
continue to work without modification. The cursor is an opaque token
encoding created_at and job_id for stable keyset pagination.

Closes leeworks-agents/SPARC#25

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

SPARC/database.py

@@ -568,20 +568,45 @@ class DatabaseClient:
         self,
         status: Optional[str] = None,
         limit: int = 10,
+        cursor: Optional[str] = None,
     ) -> List[Dict]:
-        """List jobs, optionally filtered by status."""
-        query = "SELECT * FROM jobs"
+        """List jobs with optional status filter and cursor-based pagination.
+
+        Args:
+            status: Optional status filter (pending, running, completed, failed).
+            limit: Maximum number of jobs to return.
+            cursor: Opaque cursor (``created_at|job_id``) from a previous
+                response. When provided, only jobs older than the cursor are
+                returned.
+
+        Returns:
+            List of job dicts ordered by created_at descending.
+        """
+        conditions: list[str] = []
         params: list = []
+
         if status:
-            query += " WHERE status = %s"
+            conditions.append("status = %s")
             params.append(status)
-        query += " ORDER BY created_at DESC LIMIT %s"
+
+        if cursor:
+            try:
+                ts_str, cursor_job_id = cursor.rsplit("|", 1)
+                conditions.append("(created_at, job_id) < (%s, %s)")
+                params.extend([ts_str, cursor_job_id])
+            except ValueError:
+                pass  # Ignore malformed cursors; return from start
+
+        query = "SELECT * FROM jobs"
+        if conditions:
+            query += " WHERE " + " AND ".join(conditions)
+        query += " ORDER BY created_at DESC, job_id DESC LIMIT %s"
         params.append(limit)
 
         with self.get_conn() as conn:
-            with conn.cursor(cursor_factory=RealDictCursor) as cursor:
-                cursor.execute(query, params)
-                return [dict(row) for row in cursor.fetchall()]
+            with conn.cursor(cursor_factory=RealDictCursor) as cur:
+                cur.execute(query, params)
+                return [dict(row) for row in cur.fetchall()]
 
     def mark_stale_jobs_failed(self) -> int:
         """Mark any jobs in 'running' or 'pending' state as 'failed'.