fix: back-pressure web commands and report radio failures

Web commands are answered once queued, but the main loop handed all of
them to the radio at once and anything past its free slots was dropped
silently.  Unpair also cleared paired even when the Prog wasn't sent.

- processQueue leaves commands queued until the radio has room.
- Commands that transmit get a 503 straight away during a frequency
  scan instead of being dropped later.
- Unpair restores paired when the Prog can't be sent.
- Failed web commands are logged.
- Note that /setPositions and /setSensor answer with the state before
  the command; the socket events carry the result.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/Somfy.h

@@ -575,6 +575,8 @@ class Transceiver {
     void suspendTx(uint8_t reason);
     void resumeTx(uint8_t reason);
     bool isQueued(uint32_t seq);
+    bool txSuspended();
+    bool txHasRoom();
     void beginTransmit();
     void endTransmit();
     void emitFrame(somfy_frame_t *frame, somfy_rx_t *rx = nullptr);