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>

tests/test_api_system_activity.py

@@ -182,6 +182,25 @@ async def test_runs_filter_by_status(client, _seed_runs):
     assert len(body["runs"]) == 2  # i=3, i=4
 
 
+@pytest.mark.asyncio
+async def test_runs_filter_by_task_substring(client, _seed_runs):
+    # Case-insensitive substring across the full task_name.
+    resp = await client.get("/api/system/activity/runs?task=FAKE")
+    body = await resp.get_json()
+    assert len(body["runs"]) == 5
+    assert all("fake" in r["task_name"] for r in body["runs"])
+
+
+@pytest.mark.asyncio
+async def test_runs_filter_by_task_escapes_underscore(client, _seed_runs):
+    # The literal "_" in "task_3" must match one row, not act as a
+    # single-char wildcard matching every task_N.
+    resp = await client.get("/api/system/activity/runs?task=task_3")
+    body = await resp.get_json()
+    assert len(body["runs"]) == 1
+    assert body["runs"][0]["task_name"].endswith("task_3")
+
+
 @pytest.mark.asyncio
 async def test_runs_keyset_cursor(client, _seed_runs):
     page1 = await (await client.get("/api/system/activity/runs?limit=2")).get_json()