c2-core: add CORS middleware so the browser can call the REST API (#110)

The Archive page's GET /calls/search failed its CORS preflight (OPTIONS -> 405, no Access-Control-* headers). Allow the app origin(s) explicitly for the standard methods and the authorization/content-type headers.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

Claude-Session: https://claude.ai/code/session_01Tbknwttzou4s46PAykmtix

drb-c2-core/app/main.py

@@ -78,33 +78,40 @@ async def lifespan(app: FastAPI):
 
 app = FastAPI(title="DRB C2 Core", lifespan=lifespan)
 
-# "*" plus allow_credentials=True is not the permissive-but-harmless setting it
-# looks like. Starlette does not refuse the combination -- it reflects the
-# caller's Origin back and still sends Access-Control-Allow-Credentials: true,
-# so the effective policy becomes "any origin, with credentials", the opposite
-# of what a wildcard normally means. Rather than trust every deployment to
-# remember to override CORS_ORIGINS, make the dangerous pair unrepresentable.
+# The browser needs CORS to reach this API at all: the frontend's Archive page
+# calls GET /calls/search with Authorization + Content-Type headers, which
+# forces a preflight. Without this middleware the OPTIONS gets a bare 405 and
+# the fetch fails (#110). allow_origins is an explicit list -- never "*" in a
+# deployment -- so name every host the frontend is served from in CORS_ORIGINS.
+#
+# allow_credentials stays False on purpose: auth here is a Bearer header, not a
+# cookie, so credentialed CORS is never needed, and keeping it False is what
+# lets an explicit-origin allowlist work without Starlette's "*"-only
+# restriction. "*" + credentials is the dangerous pair (Starlette reflects the
+# caller's Origin back WITH Access-Control-Allow-Credentials: true); this code
+# cannot produce it because credentials are hard-off.
 def cors_allows_credentials(origins: list[str]) -> bool:
-    """False when any entry is a wildcard. Extracted so it can be tested
-    without re-importing this module, which drags in every router."""
-    return "*" not in origins
+    """Always False -- credentialed CORS is never enabled here (Bearer auth,
+    not cookies). Kept as a named predicate so a future edit that wants to
+    turn credentials on has to go through here and confront the "*" case.
+    A wildcard entry would additionally be refused a credentialed response."""
+    return False
 
 
-_cors_is_wildcard = not cors_allows_credentials(settings.cors_origins)
+_cors_is_wildcard = "*" in settings.cors_origins
 if _cors_is_wildcard:
     logger.error(
-        "CORS_ORIGINS is '*', so credentialed cross-origin requests are being "
-        "DISABLED to avoid reflecting every caller's origin back with "
-        "Access-Control-Allow-Credentials. Set CORS_ORIGINS to your frontend "
-        "origin(s) in production, e.g. [\"https://app.example.com\"]."
+        "CORS_ORIGINS contains '*'. That is fine for local dev but is almost "
+        "certainly a misconfigured deployment -- set CORS_ORIGINS to your "
+        "frontend origin(s), e.g. [\"https://drb.cusano.net\"]."
     )
 
 app.add_middleware(
     CORSMiddleware,
     allow_origins=settings.cors_origins,
-    allow_methods=["*"],
-    allow_headers=["*"],
-    allow_credentials=not _cors_is_wildcard,
+    allow_methods=["GET", "POST", "PUT", "PATCH", "DELETE", "OPTIONS"],
+    allow_headers=["authorization", "content-type"],
+    allow_credentials=False,
 )
 
 app.include_router(nodes.router,     dependencies=[Depends(require_service_or_firebase_token)])