fix: reservation leak, spoofable delivery envelopes, and Decimal wei rendering

Seven bugs found by auditing the billing and PSS paths.

1. Reservations were never released on ack/delivery timeout. Both failure
   paths in gateway/context.ts marked the GatewayJob terminal *before* calling
   releaseReservation, and releaseReservation bailed out on exactly those
   statuses — so every failed job permanently inflated the tenant's
   reservedXbzzWei and shrank their available balance. Replace the status check
   with GatewayJob.reservationReleasedAt as an explicit compare-and-set marker,
   which makes the release exactly-once regardless of ordering, and give
   settleXbzz the same guard so a job that was released locally and claimed
   on-chain anyway can't decrement the reservation twice. The migration returns
   the xBZZ already stuck on existing accounts.

2. job_ack / job_deliver were only checked for a valid signature, never for
   *whose* signature. Anyone who learned a routing id could sign their own
   job_deliver, pointing at a chunk encrypted to the gateway's (published) PSS
   key, and hand the tenant an arbitrary completion. Drop any envelope whose
   `from` isn't the provider the job was posted to.

3. BigInt(decimal.toString()) throws on any wei value >= 1e21 (1000 xBZZ),
   because decimal.js switches to exponential notation there. That took out
   GET /accounts/me and, worse, wedged the claim watcher's settle loop. Route
   every Decimal→string conversion through toFixed().

4. A cancelJob failure at the ACK deadline left the job with no timer at all,
   so the chat request hung forever holding a semaphore slot and a reservation.
   Fall through to the delivery deadline instead.

5. `?limit=abc` reached Prisma as `take: NaN` (500). Clamp and fall back.

6. A fractional max_tokens made BigInt() throw; a negative one budgeted a
   negative reservation. Validate at the schema and defend in the estimator.

7. model.<entity>.delete() announced the deletion on the queue before
   performing it, so a failed delete still told consumers the row was gone.

Verified against a real Postgres: the reservation lifecycle (release-after-
terminal, settle-after-release, concurrent release/settle, idempotent settle)
and the migration's repair of leaked reservations on pre-fix data.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Mfkh741kNCZeBdk4xyM9qR

prisma/migrations/20260805120000_gateway_job_reservation_released_at/migration.sql

@@ -0,0 +1,27 @@
+-- AlterTable
+ALTER TABLE "GatewayJob" ADD COLUMN     "reservationReleasedAt" TIMESTAMP(3);
+
+-- Data repair: before this migration, `releaseReservation` bailed out whenever
+-- the GatewayJob row was already in a terminal status — and both failure paths
+-- in `gateway/context.ts` marked the row `cancelled` / `timed_out` *before*
+-- calling it. So every job that failed to ACK or to deliver left its
+-- `Account.reservedXbzzWei` permanently inflated, shrinking the tenant's
+-- available balance for good. Give that stuck xBZZ back, flooring at 0 so a
+-- partially-repaired database can never go negative.
+UPDATE "Account" a
+SET "reservedXbzzWei" = GREATEST(a."reservedXbzzWei" - leaked.total, 0)
+FROM (
+  SELECT j."userId" AS "userId", SUM(j."estimatedMaxXbzzWei") AS total
+  FROM "GatewayJob" j
+  WHERE j."status" IN ('cancelled', 'timed_out')
+    AND j."reservationReleasedAt" IS NULL
+  GROUP BY j."userId"
+) AS leaked
+WHERE a."userId" = leaked."userId";
+
+-- Backfill the marker for every job whose reservation is no longer outstanding
+-- (either repaired above, or already settled by `settleXbzz` on JobClaimed) so
+-- a late JobClaimed event can't decrement the same reservation twice.
+UPDATE "GatewayJob"
+SET "reservationReleasedAt" = COALESCE("claimedAt", "updatedAt")
+WHERE "status" IN ('claimed', 'cancelled', 'timed_out', 'failed');