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/Web.h

@@ -35,6 +35,7 @@ struct web_command_t {
     int8_t tiltPosition;    // for setPositions/tilt
     int8_t sunny;           // for sensor
     int8_t windy;           // for sensor
+    bool wasPaired;         // for unpair, restored if the Prog can't be sent
 };
 
 class Web {