feat(api): standardize REST contracts and auth

app/api/problem_details.py

@@ -0,0 +1,89 @@
+from __future__ import annotations
+
+from typing import Any
+from uuid import uuid4
+
+from fastapi import FastAPI, HTTPException, Request
+from fastapi.exceptions import RequestValidationError
+from fastapi.responses import JSONResponse
+from pydantic import BaseModel, Field
+
+
+class ProblemDetails(BaseModel):
+    """RFC 9457 compatible error response used by the REST contract."""
+
+    type: str
+    title: str
+    status: int
+    detail: str
+    instance: str
+    code: str
+    trace_id: str
+    errors: list[dict[str, Any]] = Field(default_factory=list)
+
+
+def _trace_id(request: Request) -> str:
+    return request.headers.get("X-Request-Id") or str(uuid4())
+
+
+def _problem_response(
+    request: Request,
+    *,
+    status_code: int,
+    title: str,
+    detail: str,
+    code: str,
+    errors: list[dict[str, Any]] | None = None,
+) -> JSONResponse:
+    problem = ProblemDetails(
+        type=f"https://tjwater.example/problems/{code.replace('_', '-')}",
+        title=title,
+        status=status_code,
+        detail=detail,
+        instance=request.url.path,
+        code=code,
+        trace_id=_trace_id(request),
+        errors=errors or [],
+    )
+    return JSONResponse(
+        status_code=status_code,
+        content=problem.model_dump(mode="json"),
+        media_type="application/problem+json",
+    )
+
+
+def install_problem_details_handlers(app: FastAPI) -> None:
+    @app.exception_handler(RequestValidationError)
+    async def validation_error_handler(
+        request: Request,
+        exc: RequestValidationError,
+    ) -> JSONResponse:
+        return _problem_response(
+            request,
+            status_code=422,
+            title="Validation error",
+            detail="Request validation failed",
+            code="validation_error",
+            errors=exc.errors(),
+        )
+
+    @app.exception_handler(HTTPException)
+    async def http_error_handler(request: Request, exc: HTTPException) -> JSONResponse:
+        detail = exc.detail if isinstance(exc.detail, str) else str(exc.detail)
+        code_by_status = {
+            401: "unauthenticated",
+            403: "forbidden",
+            404: "not_found",
+            409: "conflict",
+            422: "validation_error",
+            503: "dependency_unavailable",
+        }
+        return _problem_response(
+            request,
+            status_code=exc.status_code,
+            title=code_by_status.get(exc.status_code, "request_error")
+            .replace("_", " ")
+            .title(),
+            detail=detail,
+            code=code_by_status.get(exc.status_code, "request_error"),
+        )