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).

backend/src/mimic/app.py

@@ -3,9 +3,11 @@
 from __future__ import annotations
 
 from datetime import timedelta
+from typing import cast
 
 from flask import Flask, jsonify
 from flask.typing import ResponseReturnValue
+from werkzeug.exceptions import HTTPException
 
 from mimic.api import register_blueprints
 from mimic.auth.identity import load_user
@@ -13,12 +15,32 @@ from mimic.config import Settings, get_settings
 from mimic.extensions import db, login_manager, migrate, socketio
 from mimic.logging import configure_logging
 
+# HTTP status → stable snake_case error code surfaced in the JSON envelope.
+# Anything not listed falls back to "http_error".
+_HTTP_ERROR_CODES: dict[int, str] = {
+    400: "bad_request",
+    401: "not_authenticated",
+    403: "forbidden",
+    404: "not_found",
+    405: "method_not_allowed",
+    409: "conflict",
+    415: "unsupported_media_type",
+    422: "validation_error",
+    429: "rate_limited",
+    500: "internal_error",
+    503: "service_unavailable",
+}
+
 
 def create_app(settings: Settings | None = None) -> Flask:
     settings = settings or get_settings()
     configure_logging(settings.log_level, as_json=settings.log_json)
 
     app = Flask(__name__)
+    # `strict_slashes=False` means routes match with or without the trailing
+    # slash. Cross-origin clients keep their session cookie either way (a
+    # 308 redirect could drop it on some browsers).
+    app.url_map.strict_slashes = False
     app.config.update(
         SECRET_KEY=settings.secret_key.get_secret_value(),
         SQLALCHEMY_DATABASE_URI=settings.database_url,
@@ -43,6 +65,38 @@ def create_app(settings: Settings | None = None) -> Flask:
             401,
         )
 
+    @app.errorhandler(HTTPException)
+    def _json_http_error(exc: HTTPException) -> ResponseReturnValue:
+        """Serialize every aborted request as the uniform JSON envelope.
+
+        `flask.abort()` defaults to a Werkzeug HTML page; without this handler
+        the contract documented in docs/api.md would only hold for 401s.
+        """
+        status = exc.code or 500
+        # The Werkzeug type stub pins `description` to `str | None`, but
+        # `flask.abort(..., description=<list|dict>)` legally smuggles richer
+        # payloads through (we use this for Pydantic `errors()` on 422). Cast
+        # to `object` so the runtime type-narrowing below is type-checked.
+        description = cast(object, exc.description)
+        message: str
+        details: object | None = None
+        if isinstance(description, str):
+            message = description
+        elif isinstance(description, list | dict):
+            # Pydantic `exc.errors()` flows through `abort(422, description=...)`
+            # as a list; keep it under `details` so the client can map per-field.
+            message = "request failed"
+            details = description
+        else:
+            message = exc.name or "request failed"
+        body: dict[str, object] = {
+            "error": _HTTP_ERROR_CODES.get(status, "http_error"),
+            "message": message,
+        }
+        if details is not None:
+            body["details"] = details
+        return jsonify(body), status
+
     socketio.init_app(
         app,
         cors_allowed_origins=settings.cors_origins or "*",