Stale-PR sweep: unblock #9353 (location 4xx) DeepSource coverage gate

Follow-up from 2026-05-09 stale-PR audit §8 item 3. PR #9353 (3meeb, fix(location): return 4xx instead of 500 for invalid address input) is APPROVED + green on GitHub Actions/Vercel, blocked only by DeepSource Test coverage (apps/api) gate. Coverage delta only. REFRAMED PER LEARNINGS 2026-05-10 AI #2: original description asked 3meeb a yes/no question (add test, or accept delta?) which requires a turn-taking conversation he hasn't replied to in 24h. Replace with an offer-the-fix framing: a single in-thread ping along the lines of `I can spawn a sub-agent to add a unit test covering the 4xx branch (~30 lines). Okay to proceed, or do you want to ship as-is and accept the coverage delta?` Pre-stage the sub-agent task so the moment he says yes (or stays silent past 24h with a documented default-to-fix), execution starts. Sub-agent prep: target file is the location resolver/handler that returns the 4xx (find via PR diff). Test should cover invalid address input, assert 4xx response code + error shape. Use existing apps/api test harness conventions. Branch prefix kai/. Standard MANDATORY CONVENTIONS block. DoD: PR merged. Allowed paths: (a) 3meeb says yes → sub-agent ships test PR → 3meeb merges his PR with new test included; (b) 3meeb says no → close fleet-task with documented decision; (c) silence > 48h → close fleet-task and escalate the coverage-gate-vs-merge-decision to Carson as a policy question.