Standardize cron / Bearer auth across API routes

Problem

The codebase has two different patterns for Bearer-token-protected API routes:

1. The shared helper requireBearerSecret(request, ENV_NAME) (and its cron-specific wrapper requireCronSecret) used by every /api/sync/* route.
2. A custom local validateAuth(request) in /api/invoices/ingest/route.ts that re-implements the same pattern slightly differently.

Two implementations of the same primitive means a future hardening (e.g. constant-time comparison, audit logging on auth failure, rate limiting) has to be applied twice and can drift. It's also harder for a new contributor — or an AI agent — to discover the canonical way to add a new protected route.

Evidence

src/app/api/invoices/ingest/route.ts:21-26:

function validateAuth(request: NextRequest): boolean {
  const secret = process.env.INVOICE_INGEST_SECRET;
  if (!secret) return false;
  const authHeader = request.headers.get(\"authorization\");
  return authHeader === `Bearer ${secret}`;
}

vs the shared helper used by /api/sync/* routes (see src/lib/cron-auth.ts or wherever requireBearerSecret lives).

The local one:

• Does plain string equality (timing-attack-vulnerable; the shared helper should use timingSafeEqual if it doesn't already).
• Returns a boolean instead of returning a NextResponse | null like the shared helper, so the calling code has to write its own 401 response.

Proposed approach

1. Read src/lib/cron-auth.ts (or the canonical location of requireBearerSecret) to confirm the contract.
2. If the shared helper does not already use a constant-time comparison, fix that first as part of this issue (use crypto.timingSafeEqual after coercing to Buffer of equal length).
3. Replace validateAuth in /api/invoices/ingest/route.ts with a call to the shared helper passing \"INVOICE_INGEST_SECRET\".
4. Sweep the rest of src/app/api/**/route.ts for any other one-off Bearer checks and replace them.
5. Add a unit test for requireBearerSecret:
   • missing header → returns 401 response
   • wrong secret → returns 401
   • correct secret → returns null (i.e. "continue")
   • missing env var → returns 500 / clear error (or whatever the helper currently does — match it)

Acceptance criteria

• No route-local Bearer-validation helpers remain in src/app/api/.
• requireBearerSecret uses constant-time comparison.
• Behavior of /api/invoices/ingest is unchanged (auth still works, error responses still match).
• Unit tests cover the helper.
• pnpm lint && pnpm typecheck && pnpm test pass.

Verification

1. POST /api/invoices/ingest with no Authorization header → 401.
2. POST with wrong Bearer → 401.
3. POST with correct Bearer + valid PDF → 200 (or expected response).
4. Existing /api/sync/* routes still work unchanged.

Start by reading src/lib/cron-auth.ts and src/app/api/invoices/ingest/route.ts to confirm the shared helper contract, then sweep src/app/api/**/route.ts for other local Bearer checks. Add the specified helper tests for missing, wrong, and correct credentials, and verify with pnpm lint && pnpm typecheck && pnpm test plus the listed ingest and sync route checks.