Improve mesh media throughput (#845)

* Improve mesh media throughput

* Reserve media slots atomically

* Prioritize small fragment trains

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Co-authored-by: jack <jackjackbits@users.noreply.github.com>

bitchat/Utils/FileTransferLimits.swift

@@ -5,9 +5,9 @@ enum FileTransferLimits {
     /// Absolute ceiling enforced for any file payload (voice, image, other).
     static let maxPayloadBytes: Int = 1 * 1024 * 1024 // 1 MiB
     /// Voice notes stay small for low-latency relays.
-    static let maxVoiceNoteBytes: Int = 1 * 1024 * 1024 // 1 MiB
+    static let maxVoiceNoteBytes: Int = 512 * 1024 // 512 KiB
     /// Compressed images after downscaling should comfortably fit under this budget.
-    static let maxImageBytes: Int = 1 * 1024 * 1024 // 1 MiB
+    static let maxImageBytes: Int = 512 * 1024 // 512 KiB
     /// Worst-case size once TLV metadata and binary packet framing are included for the largest payloads.
     static let maxFramedFileBytes: Int = {
         let maxMetadataBytes = Int(UInt16.max) * 2 // fileName + mimeType TLVs