Normalise server route handlers and reply messages

[Kevinjohn]

Part of #1409 (T12). Merge predecessor: #1423 (this PR's base branch, feature/test-scaffolding).

• Error strings the client matches are a frozen wire contract in shared/src/api/replyMessages.ts; src/data/sync/lifecycleOps.ts imports the constant instead of repeating the text. Every other server message lives in server/src/routes/replyErrors.ts, in sentence case with a full stop.
• One body-parsing style (the ParseResult parser pattern), typed route generics instead of req.query/req.params casts, and every reply returned as return reply.code().send().
• New pins: the lifecycle 404 body equals the frozen text; a scoped delete with a missing or repeated accountId is refused with 400 and leaves the row. The missing-accountId test previously used a lifecycle entity and never reached that check.

Wire shapes, status codes and frozen messages are unchanged; conformance pins were updated deliberately.

Validation: Integrated Batch 4 tip (all five Batch 4 branches): pnpm run gate:all and pnpm run e2e (290 passed) on Node 24.16.0. Later changes on the stack touch only comments, the comment checker and two reference pages; on the final tip typecheck, lint, format, the comment-voice check and its test, ui:check, docs:build and pnpm run gate:server pass. Independently reviewed; findings applied. pnpm run test:account-conformance passes (302 tests).

Merge order

1. Route support to Discussions and remove PR screenshots #1411, Add project status, comparison, testing and philosophy pages #1412, Archive changelog sections before 0.41.0-alpha.3 #1414 (independent)
2. Add a first-change path for contributors #1413 (after Archive changelog sections before 0.41.0-alpha.3 #1414), Restate the comment standard as numbered voice rules #1415, Reduce configuration and hide password MFA #1416
3. Rename SMALLSASS_ACCOUNT_ settings to CAPACITYLENS_ #1417 → Add an init command that writes the environment file #1418
4. Retire unused settings and derive engagement grouping #1419 → Move diagnostics to an Owner and Admin page #1420 → Reshape the documentation into four role guides #1421 → Use @/ for app imports that leave their folder #1422 → Consolidate server test scaffolding #1423 → Normalise server route handlers and reply messages #1424 → Apply the comment-voice rules and check them in the gate #1425 (Apply the comment-voice rules and check them in the gate #1425 also after Restate the comment standard as numbered voice rules #1415) → Name the tenant boundary account in code #1426

Merge predecessor for this PR: #1423.

[Kevinjohn]

Polish review: no P1/P2 findings.

• Independent review: no client-matched message, status code or wire shape changed. Every reply returns, and no parser became stricter or looser.
• Ladder passes on Node 24: format, lint, typecheck, file sizes and server tsc, the app API, data and shared suites, and the full server suite. Four server tests (migration rehearsal and member-link concurrency) timed out under machine load and pass in isolation.

Merge order for this batch: #1411, #1412, #1414 → #1413, #1415, #1416 → #1417 → #1418, #1419 → #1420 → #1421 → #1422 → #1423 → #1424 → #1425 → #1426. Predecessor: #1423.