feat(auth): document caller header precedence + warn on conflict (#10)

CallerMiddleware silently preferred X-Auth-Request-User over X-Forwarded-User
with no explanation and no signal when both were set. Documented the precedence
(X-Auth-Request-User is the verified OIDC identity oauth2-proxy sets, so it is
authoritative; X-Forwarded-User is a fallback), and it now takes a *slog.Logger
and warns when both headers are present and disagree, so a proxy
misconfiguration is visible instead of silently resolved. Table-driven tests
cover precedence (both/single/none) and the conflict-warning path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

internal/auth/caller.go

@@ -2,17 +2,40 @@ package auth
 
 import (
 	"context"
+	"log/slog"
 	"net/http"
 )
 
 type ctxKey struct{}
 
-func CallerMiddleware(next http.Handler) http.Handler {
+// CallerMiddleware extracts the authenticated username from the reverse-proxy
+// identity headers and stashes it in the request context for Caller().
+//
+// Header precedence: X-Auth-Request-User takes priority over X-Forwarded-User.
+// X-Auth-Request-User is the header oauth2-proxy sets from the *verified* OIDC
+// identity, so it is authoritative. X-Forwarded-User is a weaker, proxy-set
+// convention some setups populate instead; it is used only as a fallback when
+// X-Auth-Request-User is absent. If a proxy sets BOTH and they disagree, the
+// verified X-Auth-Request-User still wins and we log a warning so the
+// misconfiguration is visible rather than silently resolved (#10).
+//
+// logger may be nil, in which case the conflict warning is skipped.
+func CallerMiddleware(logger *slog.Logger, next http.Handler) http.Handler {
 	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
-		user := r.Header.Get("X-Auth-Request-User")
+		authUser := r.Header.Get("X-Auth-Request-User")
+		fwdUser := r.Header.Get("X-Forwarded-User")
+
+		user := authUser
 		if user == "" {
-			user = r.Header.Get("X-Forwarded-User")
+			user = fwdUser
 		}
+
+		if logger != nil && authUser != "" && fwdUser != "" && authUser != fwdUser {
+			logger.Warn("conflicting caller identity headers; using X-Auth-Request-User",
+				"x_auth_request_user", authUser,
+				"x_forwarded_user", fwdUser)
+		}
+
 		ctx := context.WithValue(r.Context(), ctxKey{}, user)
 		next.ServeHTTP(w, r.WithContext(ctx))
 	})