feat(tier2): smarter fraud detection — Block G

Per-event fraud tuning. Hosts can now dial the medium / high / block
boundaries, allowlist trusted networks, and feed verdicts back on
flagged accesses — the seed corpus for a future ML model.

Schema (migration 0011)
- events.fraud_{medium,high,block}_threshold default 30/60/85 so
  existing events behave identically until a host changes them
- access_logs.geo_{country,city,lat,lon} for future enrichment
- fraud_feedback table — verdict ('legitimate' | 'suspicious') + note,
  PK on access_log_id so re-mark is an upsert
- event_allowlists table — (event_id, ip_cidr) primary key, inet column
  so containment checks use the native >>= operator (indexed lookup)

Domain
- FraudThresholds with Valid() + Band() helpers; Default trio echoed
  through GET responses so the frontend doesn't duplicate constants
- ParseAllowlistCIDR accepts bare IPs (auto-widens to /32 or /128) and
  canonicalises the output (203.0.113.42 → 203.0.113.42/32)
- Event.Thresholds() falls back to defaults if columns weren't
  populated yet, so the API never wedges every score into "low"

Storage
- AllowlistRepo: List / Add / Remove + Matches() — the latter pushes
  CIDR containment into Postgres rather than streaming rows back
- FeedbackRepo: Record (upserts) + ListForEvent (joined through guests)
- EventRepo.GetThresholds + UpdateThresholds, plus the threshold
  columns baked into scanEvent so every event load carries them
- AccessLogRepo.BelongsToEvent — stops a hostile editor on event A
  from marking event B's access logs

API
- GET/PUT /events/{id}/security/thresholds (viewer/editor)
- GET/POST/DELETE /events/{id}/security/allowlist
- POST /events/{id}/access-logs/{log_id}/feedback (editor)
- GET /events/{id}/security/feedback
- RSVP scoring path: allowlist short-circuit fires before the fraud
  engine; the engine's score is then re-banded against the event's
  thresholds (engine.Risk becomes advisory — API is the source of
  truth for "what counts as block here")
- CORS Allow-Methods already includes PUT (Block D fix)

Fraud engine
- Single-signal cap: it now takes ≥2 sub-scores of ≥70 to push the
  final into HIGH. Fixes the well-known "second visit with a slightly
  shifted fingerprint scores 60+" false positive
- Engine band remains advisory; API re-bands using per-event
  thresholds before deciding to block

Frontend
- SecurityCard.vue: visual band ribbon (proportional to thresholds),
  three sliders with mutual clamping so dragging medium past high
  pushes high (not an invalid ordering), reset-to-defaults button,
  CIDR allowlist with inline add + per-row remove, verdict-history
  inbox. Toast feedback on save/add/remove
- "Security" tab added to the event-detail tab nav (5th tab,
  right of Analytics)
- Viewer role hides write affordances; server enforces too

Tests
- Domain: ThresholdsBand, ThresholdsValid, ParseAllowlistCIDR (bare
  IP widening + traversal/typo rejection), FraudFeedbackValid
- Integration: thresholds round-trip + invalid ordering rejection,
  allowlist CRUD + duplicate 409 + invalid CIDR 400 + IP auto-widen,
  feedback record + upsert + cross-tenant 404 + invalid verdict 400,
  viewer can read / editor can write / outsider gets 404
- Full integration suite green (315.8s, all 36 top-level tests pass)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/pages/rsvp/[token].vue

@@ -277,10 +277,12 @@ const submitLabel = computed(() => {
             class="h-10 w-10 rounded object-contain bg-zinc-900"
           />
           <div>
+            <!-- Eyebrow text takes the accent colour when the host set
+                 one; otherwise falls back to the default brand green. -->
             <p
               class="text-xs uppercase tracking-widest"
-              :style="branding?.primary_color ? { color: 'var(--brand-primary)' } : undefined"
-              :class="branding?.primary_color ? '' : 'text-brand-500'"
+              :style="branding?.accent_color ? { color: 'var(--brand-accent)' } : undefined"
+              :class="branding?.accent_color ? '' : 'text-brand-500'"
             >
               {{ existing ? 'Update your response' : 'Invitation' }}
             </p>
@@ -293,7 +295,8 @@ const submitLabel = computed(() => {
 
         <p
           v-if="greetingMessage"
-          class="mb-4 rounded-md border border-brand-900/40 bg-brand-500/[0.04] p-3 text-sm text-zinc-300"
+          class="mb-4 rounded-r border-l-[3px] bg-brand-500/[0.04] py-1.5 pl-3 pr-2 text-sm text-zinc-300"
+          :style="branding?.accent_color ? { borderColor: 'var(--brand-accent)' } : { borderColor: 'rgb(34 197 94 / 0.6)' }"
         >{{ greetingMessage }}</p>
 
         <p class="mb-6 text-sm">