feat(api): establish stable HTTP contract — uniform {success,data} envelope, status codes, /api/v1

Point 1 of the v1.0 lock-in: commit to a stable HTTP API (the cleanest, fullest form).

Core (centralized):
- Every JSON /api response now uses ONE envelope via a Fastify preSerialization hook (src/web/server.ts): success -> { success:true, data:<payload> }; error -> { success:false, error, errorCode } with a conventional HTTP status. Non-JSON routes (file-raw, tail-file SSE, download, screenshots, /q redirect, WS) are skipped.
- Error-code -> HTTP status is a single source of truth (httpStatusForErrorCode in src/types/api.ts): 400/401/404/409/422/429/500. Expanded ApiErrorCode (added UNAUTHORIZED, CONFLICT, RATE_LIMITED). Errors are no longer HTTP 200.
- Versioned alias: /api/v1/* rewrites to /api/* (rewriteApiV1Url), so external clients pin to a stable surface while the bundled UI keeps using /api/*.
- Handlers stripped of manual 'success:true' (50 across 14 route files) so they return bare payloads the hook wraps uniformly; fixed the mux DELETE {success:<bool>} envelope collision (-> {killed}).

Frontend (48 call sites across 10 files):
- _apiJson() auto-unwraps { success:true, data } -> data (null on error), so most bare-shape readers are transparent. Raw-fetch sites relocate payload reads under .data; success/res.ok/error checks unchanged.

Docs: new docs/api-reference.md (envelope, status table, error codes, /api/v1, SSE); versioning-policy.md flipped — the HTTP/SSE API is now part of the stable, SemVer-covered surface.

Verification: full unit/route suite green (2680 passed) incl. ~166 updated assertions across 24 test files; typecheck/lint/format/frontend-syntax clean; a headless-chromium smoke loaded the migrated UI and drove the panels with 0 console/page errors; /api/status and /api/v1/status confirmed returning the uniform envelope live.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

docs/api-reference.md

@@ -0,0 +1,90 @@
+# HTTP API Reference
+
+Codeman's HTTP API is a **stable contract** as of 1.0 — see
+[`versioning-policy.md`](versioning-policy.md) for the SemVer guarantee. This page
+defines the response envelope, status codes, error codes, versioning, and the SSE
+event channel.
+
+## Versioning
+
+- The stable, public surface is served under **`/api/v1/...`**. Pin external
+  clients to this prefix.
+- The unversioned **`/api/...`** paths are a permanent alias of the current
+  version (what the bundled web UI uses). They are kept working, but new external
+  integrations should use `/api/v1`.
+- Breaking changes to the contract ship under a new prefix (`/api/v2`); `/api/v1`
+  keeps its semantics. Additive changes (new endpoints, new optional fields, new
+  error codes) are non-breaking and may appear in a minor release.
+- The implementation rewrites `/api/v1/*` → `/api/*` at the server level
+  (`rewriteApiV1Url` in `src/web/server.ts`).
+
+## Response envelope
+
+Every JSON response uses one uniform envelope, applied centrally by a
+`preSerialization` hook (`src/web/server.ts`) — handlers return bare data and the
+hook wraps it:
+
+**Success** — HTTP `2xx`:
+
+```json
+{ "success": true, "data": <payload> }
+```
+
+`data` is the endpoint's payload (object, array, or value). Endpoints with no
+payload return `{ "success": true, "data": {} }`.
+
+**Error** — HTTP `4xx`/`5xx`:
+
+```json
+{ "success": false, "error": "human-readable message", "errorCode": "NOT_FOUND" }
+```
+
+`ApiResponse<T>` in `src/types/api.ts` is the canonical type.
+
+> Non-JSON endpoints are exempt from the envelope: `GET /api/sessions/:id/file-raw`,
+> `GET /api/sessions/:id/tail-file` (SSE), `GET /api/download`,
+> `GET /api/screenshots/:name`, `GET /q/:code` (QR redirect), and the
+> `GET /ws/sessions/:id/terminal` WebSocket upgrade.
+
+## Error codes → HTTP status
+
+The single source of truth is `ErrorStatus` / `httpStatusForErrorCode()` in
+`src/types/api.ts`. Clients should branch on `errorCode` (stable) and may rely on
+the HTTP status.
+
+| `errorCode` | HTTP | Meaning |
+|-------------|------|---------|
+| `INVALID_INPUT` | 400 | Malformed request / failed validation |
+| `UNAUTHORIZED` | 401 | Authentication required or failed |
+| `NOT_FOUND` | 404 | Resource does not exist |
+| `SESSION_BUSY` | 409 | Session is busy |
+| `CONFLICT` | 409 | Conflicts with current state (e.g. already running) |
+| `ALREADY_EXISTS` | 409 | Resource already exists |
+| `OPERATION_FAILED` | 422 | Well-formed but could not be completed |
+| `RATE_LIMITED` | 429 | Too many requests |
+| `INTERNAL_ERROR` | 500 | Unexpected server error |
+
+Adding a new error code is non-breaking; removing or renaming one is a major change.
+
+## Authentication
+
+Optional HTTP Basic (`CODEMAN_USERNAME`/`CODEMAN_PASSWORD`) → opaque
+`codeman_session` cookie. When enabled, unauthenticated requests get
+`401 UNAUTHORIZED`; rate-limited requests get `429 RATE_LIMITED`. See
+[`security-architecture.md`](security-architecture.md).
+
+## SSE event channel
+
+`GET /api/events` is a Server-Sent Events stream (`text/event-stream`); each
+message is `event: <name>` + `data: <json>`. The event-name registry
+(`src/web/sse-events.ts`, mirrored in `src/web/public/constants.js`) is part of
+the stable contract — event names are not renamed without a major bump. An
+optional `?sessions=<id,...>` filter suppresses only the high-volume terminal
+stream; lifecycle/metadata events are delivered to all clients regardless.
+
+## Consuming from JavaScript
+
+The bundled frontend reads responses through `_apiJson()`
+(`src/web/public/api-client.js`), which unwraps `{success:true,data}` → `data` and
+returns `null` on a non-2xx / `{success:false}` response. External clients should
+do the same: check the HTTP status (or `body.success`), then read `body.data`.