fix: review round 7 — fresh-eyes audit fixes (6 agents) + full test suites for every backend change

Fresh-eyes review round with 6 independent agents (money-safety, concurrency,
Square wire parity, security, frontend flow, testing-gaps). Every finding was
independently verified against the code before fixing. All backend changes
now carry full test suites (10+ new tests, each verified to FAIL without its
guard). All 20 packages green, race detector clean.

Money-safety:
- Gift-card purchase refunds no longer create money: manual refunds of a
  no-booking (gift-card purchase) payment are rejected with a clear message
  in the direct handler AND never re-issued by the sweep-resume path
  (processManualPaymentGroup skips them; reconcile-then-fail, no re-issue).
- BuyGiftCard no-client-key fallback: derived deterministically under the
  advisory lock (pending-row reuse fixes lost-response double-charge;
  completed-row sequence advance preserves distinct-purchase collapse fix).
- Terminal completion is never unrecorded: activeTerminalCheckoutID now calls
  recordUntrackedTerminalPayment when a provisional (tmp-) checkout is found
  COMPLETED at Square (previously only marked the row COMPLETED — a lost poll
  left the payment invisible and unrefundable).
- Sweep: provisional tmp- checkout rows are resolved against Square first
  (COMPLETED → record; live → keep guard; NOT_FOUND/CANCELED → fail;
  ambiguous → leave pending) instead of blind-failing a possibly-live
  checkout. recordUntrackedTerminalPayment re-checks the booking status
  (FOR UPDATE) and refuses to record on a cancelled booking, inserting a
  critical_payment_log admin notification instead. Till-sale post-charge
  UPDATE now requires status='pending' (no resurrection of a clawed-back sale).

Frontend (Svelte 5):
- UserPaymentModal keeps CardSelection mounted through processing (bind:this
  ref + Square iframe survive the loyalty/tokenize awaits) — new-card
  payments work again.
- BookingFlow clears the cached nonce/verification pair on any failure (retry
  re-tokenizes fresh; idempotency key retained for dedup); 409 'already paid'
  refetches the booking and reconciles depositPaid so the confirmation gate
  opens; Back button disabled during processing.
- Synchronous double-submit guards on buyGiftCard/redeemGiftCard/submitTip.

Square wire parity (mock vs real):
- processing_fee sign unified (negated at paymentFromSquare; mock agrees).
- SimulateSourceUsed (SOURCE_USED, 400) matches real CreateCard.
- GetCardsOnFile excludes disabled cards (matches ListCards).
- ForcePaymentStatus toggle + tests prove the charge path can't be status-blind.
- CreateCheckout rejects empty device_id (env fallback SQUARE_TERMINAL_DEVICE_ID);
  completed terminal checkout's payment resolvable by id.

Security:
- 2FA attempt-map data race fixed: lastAt is atomic.Int64 (nanos) — eviction
  scan reads race-free; concurrent verify+evict tests under -race.
- Backend refuses to start on weak/placeholder JWT_SECRET_KEY (<32 chars or
  known public placeholders) with openssl rand -hex 32 guidance.
- Dockerfile no longer COPYs .env (secrets injected via compose env_file).
- SabreDAV requires DAV_ADMIN_PASSWORD (no admin/admin default); compose
  fails at config time when missing.

Testing gaps closed (each verified to FAIL without its guard):
- refunded-dedup 409 (CreateBookingPayment), keyed sweep past-retention
  blind-fail, reconcile status-switch (CANCELED/FAILED/APPROVED/PENDING/unknown
  in both by-key and by-id paths), resolveChargeSource Square-failure branches,
  structured 500 / CARD_DECLINED / cancelled-context E2E (row stays pending),
  deriveBookingPaymentIdempotencyKey >45-char truncation, webhook
  findPaymentByDisputeID fallback, clawbackOneTillSale non-gift-card branch,
  dispute.evidence / terminal.checkout dispatch.

Infra:
- local-dev-2.sh fails loudly on port-5432 squatters / docker compose failures
  (previously died silently under ERR_EXIT with hidden output).
- Test harness defaults SQUARE_TERMINAL_DEVICE_ID; money_safety_fixes_test.go
  gained the missing build tag.

Verification: go test -tags test,dev -count=1 -parallel 8 ./... (20/20 ok),
-race clean on 2FA + payments money paths, go build ./... + -tags dev, go vet
clean, svelte-check 0 errors, env-docs gate OK (36 vars), docker compose
config valid.

sabredav/server.php

@@ -144,8 +144,17 @@ if ($stmt->fetchColumn() == 0) {
     ");
 }
 
-// Create default user if not exists (username: admin, password from DAV_ADMIN_PASSWORD env)
-$davPassword = getenv('DAV_ADMIN_PASSWORD') ?: 'admin';
+// Create default user if not exists (username: admin, password from DAV_ADMIN_PASSWORD env).
+// DAV_ADMIN_PASSWORD is REQUIRED: this CardDAV/CalDAV server exposes customer
+// PII (vCards), so a default or publicly-known admin credential is never
+// acceptable — fail fast instead of starting with one.
+$davPassword = getenv('DAV_ADMIN_PASSWORD');
+$weakDavPasswords = ['admin', 'password', 'changeme', 'change-me', 'changethis', 'secret', 'sabredav', 'test'];
+if ($davPassword === false || $davPassword === '' || in_array(strtolower(trim($davPassword)), $weakDavPasswords, true)) {
+    error_log("FATAL: DAV_ADMIN_PASSWORD is not set or is a known weak/default value. Refusing to start: set a strong random DAV_ADMIN_PASSWORD (e.g. `openssl rand -hex 32`) in the environment and restart.");
+    http_response_code(500);
+    die("DAV_ADMIN_PASSWORD is not configured");
+}
 $stmt = $pdo->query("SELECT COUNT(*) FROM dav_users");
 if ($stmt->fetchColumn() == 0) {
     $digest = md5('admin:SabreDAV:' . $davPassword);