feat: replace static API token with per-request Gitea PAT pass-through

Callers now supply their own Gitea PAT as a Bearer token; the server validates
it against GET /api/v1/user and threads it through context to all downstream
Gitea API calls. GITEA_API_TOKEN env var and the GiteaAPIToken config field are
removed.

internal/auth/bearer.go

@@ -0,0 +1,49 @@
+package auth
+
+import (
+	"context"
+	"net/http"
+	"strings"
+	"time"
+)
+
+type tokenKey struct{}
+
+// BearerMiddleware validates the incoming bearer token as a Gitea PAT by
+// calling GET /api/v1/user. The validated token is stored in context for
+// downstream use by the Gitea client.
+func BearerMiddleware(giteaBaseURL string, next http.Handler) http.Handler {
+	hc := &http.Client{Timeout: 5 * time.Second}
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		token, ok := strings.CutPrefix(r.Header.Get("Authorization"), "Bearer ")
+		if !ok || token == "" {
+			http.Error(w, "unauthorized", http.StatusUnauthorized)
+			return
+		}
+		req, err := http.NewRequestWithContext(r.Context(), http.MethodGet, giteaBaseURL+"/api/v1/user", nil)
+		if err != nil {
+			http.Error(w, "unauthorized", http.StatusUnauthorized)
+			return
+		}
+		req.Header.Set("Authorization", "token "+token)
+		resp, err := hc.Do(req)
+		if err != nil || resp.StatusCode != http.StatusOK {
+			if resp != nil {
+				_ = resp.Body.Close()
+			}
+			http.Error(w, "unauthorized", http.StatusUnauthorized)
+			return
+		}
+		_ = resp.Body.Close()
+		ctx := context.WithValue(r.Context(), tokenKey{}, token)
+		next.ServeHTTP(w, r.WithContext(ctx))
+	})
+}
+
+// TokenFromContext returns the validated Gitea PAT stored by BearerMiddleware.
+func TokenFromContext(ctx context.Context) string {
+	if v, ok := ctx.Value(tokenKey{}).(string); ok {
+		return v
+	}
+	return ""
+}