Fix restore-path main↔bleQueue deadlock and courier drop amplification

A device froze permanently in a two-phone test. Debugger stacks showed an
ABBA deadlock: the main actor was in bleQueue.sync (delivery-ack send →
broadcastPacket → readLinkState) while bleQueue was in main.sync
(captureBluetoothStatus reading backgroundTimeRemaining). The load that
lined the two edges up came from a courier-drop amplification storm:
drop dedup was in-memory only while the outbox driving 120s re-deposits
is persisted, so every relaunch republished the same undelivered DM as a
fresh 24h relay drop and every gateway relaunch re-fetched the whole
backlog — ~20 copies of one DM delivered in 40ms, each triggering
decrypt + delivery + ack + handshake work.

Fixes, in rank order:
- Edge B (P0): captureBluetoothStatus no longer main.syncs from bleQueue;
  backgroundTimeRemaining is sampled on main and cached behind a lock.
  Invariant documented: bleQueue must NEVER sync-dispatch to main.
- Edge A (P0, defense in depth): sendDeliveryAck / sendReadReceipt /
  sendPrivateMessage / sendNoisePayload / triggerHandshake hop to
  messageQueue like sendMessage, so no main-actor call path reaches
  readLinkState's bleQueue.sync.
- Drop dedup (P1): publishedDropKeys and seenDropEventIDs persist across
  relaunches (new BridgeDropDedupStore, entries expire with the 24h
  NIP-40 drop window; wiped on panic) — one drop per message ID per 24h
  regardless of relaunch count.
- Receiver dedup (P1): openCourierEnvelope dedups on the inner private
  message ID before delivery, so a duplicate copy costs one decrypt and
  never re-delivers, re-acks, or re-triggers a handshake.
- Handshake gating (P2): queued acks initiate a Noise handshake only for
  reachable peers; mail from absent/rotated identities no longer turns
  each copy into a mesh-wide handshake flood (the ack stays queued and
  flushes when a session eventually establishes).
- Outbox (P3): re-enqueueing a queued message ID carries over its
  depositedCourierKeys so resends stop re-burning the same courier slots.

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

bitchatTests/Services/MessageRouterTests.swift

@@ -446,6 +446,34 @@ struct MessageRouterTests {
         #expect(transport.sentCourierMessages.count == 1)
     }
 
+    @Test @MainActor
+    func enqueueReplacementCarriesOverDepositedCourierKeys() async {
+        // Re-sending a queued message ID replaces the outbox entry; the
+        // replacement must inherit which couriers already carry the message,
+        // or the deposit retry re-burns the same courier slots (duplicate
+        // sealed copies to the same peer).
+        let recipient = PeerID(str: "00000000000000aa")
+        let recipientKey = Data(repeating: 0xBB, count: 32)
+        let courier = PeerID(str: "00000000000000cc")
+        let courierKey = Data(repeating: 0xCC, count: 32)
+
+        let transport = MockTransport()
+        transport.connectedPeers.insert(courier)
+        transport.updatePeerSnapshots([Self.snapshot(courier, key: courierKey, verified: true)])
+
+        let router = MessageRouter(
+            transports: [transport],
+            courierDirectory: Self.directory(recipient: recipient, recipientKey: recipientKey)
+        )
+        router.sendPrivate("Hello", to: recipient, recipientNickname: "Peer", messageID: "ck1")
+        #expect(transport.sentCourierMessages.count == 1)
+
+        // Same message ID re-sent (e.g. a resend while still queued): the
+        // courier already carrying it must not receive a second copy.
+        router.sendPrivate("Hello", to: recipient, recipientNickname: "Peer", messageID: "ck1")
+        #expect(transport.sentCourierMessages.count == 1)
+    }
+
     @Test @MainActor
     func courierBecameAvailable_ignoresTheRecipientThemselves() async {
         let recipient = PeerID(str: "00000000000000aa")