openspec: scan-alert-hardening change — different-finder re-alert + fingerprint 24h block

openspec/changes/scan-alert-hardening/tasks.md

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+## 1. Schema & Queries
+
+- [ ] 1.1 `db/schema.sql`: idempotent `ALTER TABLE scans ADD COLUMN IF NOT EXISTS fingerprint TEXT;`; `make db-up`
+- [ ] 1.2 `queries.sql`: add `GetRecentScanByFingerprint` (:one, fingerprint + 24 h window); `make generate`; build
+
+## 2. Fingerprint JS
+
+- [ ] 2.1 `tag-public.html`: compute fingerprint (UA/language/timezone/screen/platform hash), send with scan POST and contact form (hidden input)
+
+## 3. Alert Logic
+
+- [ ] 3.1 `scan.go`: `ScanRequest` gains `phone` + `fingerprint`; `shouldAlert` checks fingerprint block first, then different-phone re-alert within window; pass fingerprint/phone into `InsertScan`
+- [ ] 3.2 `FinderContact`: accept + store fingerprint; dedup by phone within window; skip SMS when same phone or fingerprint-blocked
+
+## 4. Verification
+
+- [ ] 4.1 Unit-style checks via HTTP suite: same fingerprint within 24 h → no alert; fresh fingerprint → alert; different phone at same spot within window → alert; same phone re-submit → no alert
+- [ ] 4.2 Existing suites still pass (Phase 1 + Phase 2) on fresh DB
+- [ ] 4.3 `openspec validate scan-alert-hardening`; commit