feat(V2): bearer auth middleware con compare_digest

Implementa install_auth_middleware con whitelist /health /apidocs /openapi.json,
token timing-safe via secrets.compare_digest, request.state.environment injection.
Fix pyproject: --import-mode=prepend (importlib + PEP563 rompe FastAPI Request injection).
Rimosso from __future__ import annotations da test_auth.py per stesso motivo.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/cerbero_mcp/auth.py

@@ -0,0 +1,60 @@
+"""Bearer auth middleware: bearer token → request.state.environment."""
+from __future__ import annotations
+
+import secrets
+from typing import Literal
+
+from fastapi import FastAPI, Request, status
+from fastapi.responses import JSONResponse
+
+Environment = Literal["testnet", "mainnet"]
+
+WHITELIST_PATHS = frozenset({"/health", "/apidocs", "/openapi.json"})
+
+
+def _extract_bearer(auth_header: str) -> str | None:
+    if not auth_header.startswith("Bearer "):
+        return None
+    token = auth_header[len("Bearer "):].strip()
+    return token or None
+
+
+def _check_token(
+    candidate: str, testnet_token: str, mainnet_token: str
+) -> Environment | None:
+    if secrets.compare_digest(candidate, testnet_token):
+        return "testnet"
+    if secrets.compare_digest(candidate, mainnet_token):
+        return "mainnet"
+    return None
+
+
+def install_auth_middleware(
+    app: FastAPI,
+    *,
+    testnet_token: str,
+    mainnet_token: str,
+) -> None:
+    """Registra middleware di auth bearer sull'app FastAPI."""
+
+    @app.middleware("http")
+    async def auth_middleware(request: Request, call_next):
+        if request.url.path in WHITELIST_PATHS:
+            return await call_next(request)
+
+        token = _extract_bearer(request.headers.get("Authorization", ""))
+        if token is None:
+            return JSONResponse(
+                status_code=status.HTTP_401_UNAUTHORIZED,
+                content={"error": {"code": "UNAUTHORIZED",
+                                   "message": "missing or malformed bearer token"}},
+            )
+        env = _check_token(token, testnet_token, mainnet_token)
+        if env is None:
+            return JSONResponse(
+                status_code=status.HTTP_401_UNAUTHORIZED,
+                content={"error": {"code": "UNAUTHORIZED",
+                                   "message": "invalid token"}},
+            )
+        request.state.environment = env
+        return await call_next(request)