fix(gate): --brief leaves the test database's own log out of an excerpt

The DB lane provokes constraint errors on purpose, and their ERROR: lines
from the testdb container came first in a red pytest excerpt, ahead of the
failing test (seen in E35 S3). The excerpt now skips the testdb-N lines.

frontend/src/lib/gates.snapshot.json

@@ -1392,7 +1392,8 @@
       "}",
       "brief_excerpt() {",
       "  local hits",
-      "  hits=\"$(grep -E -i -A 2 -m 20 'error|fail|✗|×|warn|would reformat|unused|vulnerab|found [0-9]' \"$1\")\"",
+      "  hits=\"$(grep -v -E '^testdb-[0-9]+ ' \"$1\" |",
+      "    grep -E -i -A 2 -m 20 'error|fail|✗|×|warn|would reformat|unused|vulnerab|found [0-9]')\"",
       "  if [ -n \"$hits\" ]; then printf '%s\\n' \"$hits\"; else tail -n 20 \"$1\"; fi",
       "}",
       "ruff_pinned() {",