fix: 用 FetchGate actor 修复并发触发丢失问题

Sources/Services/PollerService.swift

@@ -11,8 +11,8 @@ final class PollerService {
     private let configService = AppStateService.shared
 
     private var timer: Timer?
-    private var isFetching = false
     private var lastFetchTime: Date?
+    private let fetchGate = FetchGate()
 
     private(set) var isRunning = false
 
@@ -85,11 +85,53 @@ final class PollerService {
 
     // MARK: - Fetch
 
-    private func performFetch() async {
-        guard !isFetching else { return }
-        isFetching = true
-        defer { isFetching = false }
+    /// Serializes fetches and coalesces concurrent triggers.
+    ///
+    /// Previously a trigger arriving mid-fetch was silently dropped
+    /// (`guard !isFetching else { return }`). Rapid successive changes —
+    /// e.g. an A→B→A reassignment producing two SSE wake-ups — then
+    /// collapsed into a single fetch, and if that fetch sampled the final
+    /// state the transition was never observed and no notification fired.
+    /// Now a trigger arriving mid-fetch queues exactly one follow-up round,
+    /// so the final state is always observed. The actor also makes the
+    /// check-then-set race-safe (triggers arrive from the timer, the SSE
+    /// handler and the UI on different tasks).
+    private actor FetchGate {
+        private var fetching = false
+        private var pending = false
 
+        /// Returns true if the caller may start fetching; otherwise queues
+        /// a follow-up round and returns false.
+        func tryBegin() -> Bool {
+            if fetching {
+                pending = true
+                return false
+            }
+            fetching = true
+            return true
+        }
+
+        /// Ends one round. Returns true when another round was requested
+        /// while fetching; the gate stays held between rounds so no
+        /// parallel fetch can slip in.
+        func endRound() -> Bool {
+            if pending {
+                pending = false
+                return true
+            }
+            fetching = false
+            return false
+        }
+    }
+
+    private func performFetch() async {
+        guard await fetchGate.tryBegin() else { return }
+        repeat {
+            await fetchOnce()
+        } while await fetchGate.endRound()
+    }
+
+    private func fetchOnce() async {
         lastFetchTime = Date()
 
         await MainActor.run {