feat(activity): search/filter on both Activity-tab panes

Recent failures gains a client-side search over the already-loaded 24h
rows (task/queue/target/error), shown as a filtered/total count alongside
the existing error-type chips. All recent activity gains a debounced
server-side task-name search (new `task` ILIKE param on /runs) so it
spans the full history, not just the loaded page. LIKE wildcards are
escaped so task names' literal underscores match literally.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/api/system_activity.py

@@ -147,6 +147,7 @@ async def list_runs():
     """Paginated task_run history. Query params:
       queue=<name>     filter to one queue
       status=<status>  filter to one status (running/ok/error/timeout/retry)
+      task=<substr>    case-insensitive substring match on task_name
       limit=<int>      default 50, max 200
       before_id=<int>  cursor for keyset pagination
 
@@ -161,6 +162,7 @@ async def list_runs():
 
     queue = request.args.get("queue")
     status = request.args.get("status")
+    task = request.args.get("task")
     before_id_raw = request.args.get("before_id")
     before_id = int(before_id_raw) if before_id_raw else None
 
@@ -170,6 +172,11 @@ async def list_runs():
             stmt = stmt.where(TaskRun.queue == queue)
         if status:
             stmt = stmt.where(TaskRun.status == status)
+        if task:
+            # Task names contain literal underscores (download_source,
+            # vacuum_analyze) — escape LIKE wildcards so a search for
+            # "vacuum_analyze" doesn't treat "_" as a single-char match.
+            stmt = stmt.where(TaskRun.task_name.ilike(f"%{_escape_like(task)}%", escape="\\"))
         if before_id is not None:
             stmt = stmt.where(TaskRun.id < before_id)
         stmt = stmt.limit(limit + 1)
@@ -225,6 +232,12 @@ async def list_failures():
     })
 
 
+def _escape_like(value: str) -> str:
+    """Escape SQL LIKE/ILIKE metacharacters so user search text is matched
+    literally. Pairs with `escape="\\"` on the .ilike() call."""
+    return value.replace("\\", "\\\\").replace("%", "\\%").replace("_", "\\_")
+
+
 def _row_to_dict(r: TaskRun) -> dict:
     return {
         "id": r.id,