docs: add MkDocs Material documentation site

docs/api/error-responses.md

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+# Error Responses
+
+All endpoints return consistent error responses:
+
+```json
+{
+  "status": "error",
+  "error_code": "INVALID_PARAMETERS",
+  "message": "Invalid vcodec: XYZ. Must be one of: H264, H265, VP9, AV1, VC1, VP8",
+  "timestamp": "2026-02-27T18:00:00.000000+00:00",
+  "details": { }
+}
+```
+
+Common error codes:
+
+- `INVALID_INPUT` -- malformed request body
+- `INVALID_PARAMETERS` -- invalid parameter values
+- `MISSING_SERVICE` -- service tag not provided
+- `INVALID_SERVICE` -- service not found or not in the caller's allowlist
+- `SERVICE_ERROR` -- service initialization or runtime error
+- `AUTH_FAILED` -- authentication failure
+- `NOT_FOUND` / `TRACK_NOT_FOUND` / session not found -- job/session/track/title missing
+- `INTERNAL_ERROR` -- unexpected server error
+
+When `--debug-api` is enabled, error responses include additional `debug_info` with tracebacks and stderr output.
+
+Authentication errors from the auth middleware are returned as `{"status": 401, "message": "..."}` (not the standard error envelope).