chore(sync): close stack feature and record api error contract

AGENTS.md

@@ -63,6 +63,7 @@ Stored in `docs/specs/`. Format: `docs/specs/NNNN-title/index.md`, with `rationa
 - comments only when necessary and single line only
 - Domain writes go through `backend/src/lib/repository.ts` (UUID v4 ids from `crypto.randomUUID`, an Activity row for created and updated, closed set columns validated against `backend/src/lib/constants.ts`); default reads filter `deletedAt IS NULL`, and reads that cross a relation filter soft deleted parents
 - Day boundaries come from `backend/src/lib/timezone.ts`, which reads the `TIMEZONE` env var (IANA zone, default UTC, an unparseable value fails config load); no other code builds a Date for domain logic
+- API errors answer as JSON `{ error }` from the central handler in `backend/src/app.ts`: a throw in a route becomes that body, a 4xx keeps its message, a 500 is logged by pino and returns "Internal server error" with no internals, so routes do not need their own catch for the envelope
 
 ## Git
 

docs/scope/scope.md

@@ -13,7 +13,7 @@ _These are recommendations to keep your build orderly, not requirements. Skip an
 |---|---------|-------|--------|
 | A | Client scaffold | Foundation | existing |
 | B | Backend scaffold | Foundation | in-progress |
-| 18 | Stack & architecture | Foundation | in-progress |
+| 18 | Stack & architecture | Foundation | done |
 | 1 | Coding standards & tooling | Foundation | done |
 | 2 | Data model | Foundation | done |
 | 3 | Design system & UI foundation | Foundation | planned |
@@ -40,7 +40,7 @@ Pre workflow Next.js app: default start page, Tailwind, ESLint, and its own Next
 ### B. Backend scaffold · in-progress
 Express and TypeScript started but not running yet: the dev script points at a missing `src/index.ts` and both source files are empty, so it does not boot. Finishing it is the first build job, via `/develop`. code in `backend/`
 
-### 18. Stack & architecture · in-progress
+### 18. Stack & architecture · done
 Execute the stack decision from spec 0001: one git repo at the root, both apps booting locally on the decided services, the database migrated, auth mounted, and the proxy working.
 **Done when:** both apps run locally against Neon with migrations applied, Better Auth sign in works through the proxy, and the first deploy targets Vercel and Render with the environment variables documented.
 - [x] Design it (spec): `/architect stack & architecture`