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>

internal/domain/fraud_v2_test.go

@@ -0,0 +1,86 @@
+package domain
+
+import "testing"
+
+func TestThresholdsBand(t *testing.T) {
+	tt := DefaultThresholds() // 30/60/85
+	cases := map[int]string{
+		0: "low", 15: "low", 29: "low",
+		30: "medium", 45: "medium", 59: "medium",
+		60: "high", 70: "high", 84: "high",
+		85: "block", 99: "block", 100: "block",
+	}
+	for score, want := range cases {
+		if got := tt.Band(score); got != want {
+			t.Errorf("Band(%d) = %q, want %q", score, got, want)
+		}
+	}
+}
+
+func TestThresholdsValid(t *testing.T) {
+	ok := []FraudThresholds{
+		{0, 0, 0},
+		{30, 60, 85},
+		{10, 50, 90},
+		{50, 50, 50}, // equality at every boundary is allowed
+		{100, 100, 100},
+	}
+	for _, th := range ok {
+		if err := th.Valid(); err != nil {
+			t.Errorf("expected %+v to be valid, got %v", th, err)
+		}
+	}
+	bad := []FraudThresholds{
+		{60, 30, 85}, // medium > high
+		{30, 85, 60}, // high > block
+		{-1, 30, 60}, // negative
+		{30, 60, 101},
+	}
+	for _, th := range bad {
+		if err := th.Valid(); err == nil {
+			t.Errorf("expected %+v to be invalid", th)
+		}
+	}
+}
+
+func TestParseAllowlistCIDR(t *testing.T) {
+	cases := []struct {
+		in, want string
+		ok       bool
+	}{
+		{"203.0.113.0/24", "203.0.113.0/24", true},
+		{"203.0.113.42", "203.0.113.42/32", true},   // bare IPv4 → /32
+		{"2001:db8::/32", "2001:db8::/32", true},
+		{"::1", "::1/128", true},                     // bare IPv6 → /128
+		{"not-an-ip", "", false},
+		{"", "", false},
+		{"999.0.0.0/24", "", false},
+	}
+	for _, tc := range cases {
+		got, _, err := ParseAllowlistCIDR(tc.in)
+		if tc.ok {
+			if err != nil {
+				t.Errorf("ParseAllowlistCIDR(%q) unexpected err: %v", tc.in, err)
+				continue
+			}
+			if got != tc.want {
+				t.Errorf("ParseAllowlistCIDR(%q) = %q, want %q", tc.in, got, tc.want)
+			}
+		} else if err == nil {
+			t.Errorf("ParseAllowlistCIDR(%q) should have rejected", tc.in)
+		}
+	}
+}
+
+func TestFraudFeedbackValid(t *testing.T) {
+	for _, v := range []string{"legitimate", "suspicious"} {
+		if err := (FraudFeedback{Verdict: v}).Valid(); err != nil {
+			t.Errorf("verdict %q should be valid: %v", v, err)
+		}
+	}
+	for _, v := range []string{"", "fraud", "ok", "LEGITIMATE"} {
+		if err := (FraudFeedback{Verdict: v}).Valid(); err == nil {
+			t.Errorf("verdict %q should be invalid", v)
+		}
+	}
+}