fix(backend): JSON error envelope for every HTTPException + strict_slashes=False

Two issues spotted by ux-frontend consuming docs/api.md against the actual
code path:

1. `flask.abort(...)` returned the Werkzeug HTML error page for 400/403/404/
   422/etc. — only the 401 paths going through `api_error()` and the
   Flask-Login `unauthorized_handler` honoured the `{error, message}`
   envelope the contract promised. The frontend's `ApiClientError.body`
   parser was forced to fall back to a raw string, and the 422 case
   could not surface Pydantic per-field errors.

   Fix: register `@app.errorhandler(HTTPException)` that serialises every
   `HTTPException` to the same JSON envelope. 422s gain a `details: [...]`
   field holding the Pydantic `errors()` list (`loc` / `msg` / `type`),
   matching the shape now documented in `docs/api.md`.

   A `_HTTP_ERROR_CODES` map maps statuses to stable snake_case codes
   (`bad_request`, `not_found`, `method_not_allowed`,
   `validation_error`, `forbidden`, `internal_error`, ...). Unknown
   statuses fall back to `http_error`.

   `description` is `cast(object, ...)` because the Werkzeug stub pins it
   to `str | None` while `flask.abort(..., description=<list>)` is the
   officially supported way to smuggle a Pydantic errors list to the
   handler.

2. `@bp.get("")` on the engagements blueprint produced `/api/v1/engagements`
   (no slash). Hitting it with a trailing slash issued a 308 redirect,
   and some browsers drop the session cookie across that hop.

   Fix: `app.url_map.strict_slashes = False`. Both forms now match the
   same handler without redirect.

5 new integration tests cover the new envelope shape (422 with details,
unknown 404, malformed-JSON 400) and the dual-slash matching. `docs/api.md`
rewritten to reflect the table of stable codes, the `details` shape, and
the no-trailing-slash convention. `CHANGELOG.md` gains a follow-up entry.

Verification: ruff check / mypy --strict / pytest tests/unit all green
(61 unit + 5 new integration).

docs/api.md

@@ -8,26 +8,59 @@ same origin).
 
 ## Conventions
 
-- **Base URL**: `/api/v1/`.
+- **Base URL**: `/api/v1`.
+- **Trailing slash**: routes accept the URL with **or without** a trailing
+  slash. The app is configured with `strict_slashes=False`, so Werkzeug
+  never issues a 308 redirect (which can drop the session cookie on some
+  browsers). Use whichever form your client prefers; `docs/api.md` writes
+  the no-slash form.
 - **Auth transport**: Flask session cookie (`HttpOnly`, `SameSite=Lax`,
-  `Secure` in production). The browser must send `credentials: "include"` on
-  every request.
+  `Secure` in production). The browser must send `credentials: "include"`
+  on every request.
 - **Content negotiation**: requests and responses use `application/json`.
-- **Error envelope**: failures return `{"error": "<code>", "message": "<human>"}`
-  with the appropriate HTTP status. Codes are stable identifiers (snake_case),
-  messages are human-readable but not localized.
+- **Error envelope**: **every** failure returns the same shape, served by a
+  global `HTTPException` handler:
+  ```json
+  { "error": "<snake_case_code>", "message": "<human>", "details": ... }
+  ```
+  `details` is only present for `422` (Pydantic per-field error list). Codes
+  are stable identifiers; messages are human-readable but not localized.
 
-| Status | Use |
-|--------|-----|
-| 200 | OK |
-| 201 | Resource created |
-| 204 | OK, no body |
-| 400 | Malformed request (e.g. missing JSON body) |
-| 401 | `not_authenticated` or `invalid_credentials` |
-| 403 | `forbidden` — authenticated but missing permission |
-| 404 | Resource not found (also returned for tenant-scope denials, see below) |
-| 422 | Pydantic validation error |
-| 500 | Internal — opaque, no leak |
+| Status | `error` code | Use |
+|--------|--------------|-----|
+| 200 | — | OK |
+| 201 | — | Resource created |
+| 204 | — | OK, no body |
+| 400 | `bad_request` | Malformed request (e.g. missing JSON body) |
+| 401 | `not_authenticated` or `invalid_credentials` | Anonymous / bad creds |
+| 403 | `forbidden` | Authenticated but missing permission |
+| 404 | `not_found` | Resource not found (also tenant-scope denials, see below) |
+| 405 | `method_not_allowed` | Method not allowed for that route |
+| 415 | `unsupported_media_type` | Wrong `Content-Type` on a body |
+| 422 | `validation_error` | Pydantic — see `details` |
+| 429 | `rate_limited` | Reserved for future limiter |
+| 500 | `internal_error` | Opaque — no leak |
+
+### 422 `details` shape
+
+`details` is the raw Pydantic `errors()` list — one entry per failed field:
+
+```json
+{
+  "error": "validation_error",
+  "message": "request failed",
+  "details": [
+    {
+      "type": "missing",
+      "loc": ["client_name"],
+      "msg": "Field required",
+      "input": { "description": "no client_name" }
+    }
+  ]
+}
+```
+
+Use `details[i].loc` to map the error back to a form field.
 
 ### Tenant scope leak prevention (MA6 — F11)
 
@@ -92,7 +125,7 @@ calls this at boot to rehydrate the application state.
 
 ## Engagements
 
-### `GET /api/v1/engagements/`
+### `GET /api/v1/engagements`
 
 Lists engagements visible to the caller (`engagement.read` permission).
 
@@ -115,12 +148,12 @@ Response — `200`:
 ]
 ```
 
-### `GET /api/v1/engagements/<eid>`
+### `GET /api/v1/engagements<eid>`
 
 Same payload shape as the list element. Returns 404 if the engagement does not
 exist or the caller is not a member (MA6).
 
-### `POST /api/v1/engagements/`
+### `POST /api/v1/engagements`
 
 Creates an engagement (`engagement.create` permission).
 
@@ -153,6 +186,6 @@ authority.
    ```
 2. `POST /api/v1/auth/login` with the credentials — receive the user payload
    plus the session cookie.
-3. `POST /api/v1/engagements/` with a body — receive the engagement.
-4. `GET /api/v1/engagements/` — see the new engagement in the list.
+3. `POST /api/v1/engagements` with a body — receive the engagement.
+4. `GET /api/v1/engagements` — see the new engagement in the list.
 5. `POST /api/v1/auth/logout` — session cleared.