Server: FCM HTTP v1 target in fid instead of deprecated token (D-083 follow-up)

The stored fcm_token is the Firebase Installation ID; HTTP v1 deprecates
message.token (accepted only during the transition) in favour of message.fid.
Push examples and push_fcm tests follow.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TQd3pEP9UeN2uifbHknQ7B

server/crates/kvitt-server/src/push/fcm.rs

@@ -113,10 +113,12 @@ pub fn entity_type_of(push_type: PushType) -> &'static str {
 }
 
 /// The HTTP v1 request body for one device (SPEC §14.6): data only, every value a string, ids only (AC-13.8).
+/// The target is `fid`: the stored `fcm_token` is the Firebase Installation ID the phone registered (D-083), and
+/// HTTP v1 deprecates `token` in favour of `fid` (D-096).
 pub fn message_body(fcm_token: &str, message: &PushMessage) -> Value {
     json!({
         "message": {
-            "token": fcm_token,
+            "fid": fcm_token,
             "android": { "priority": "high", "ttl": MESSAGE_TTL },
             "data": {
                 "v": PAYLOAD_VERSION,