fix: improve startup error reporting with full stderr logging and dependency pre-check

Radiosonde route now runs a quick import check before launching the full
subprocess, catching missing Python dependencies immediately with a clear
message instead of a truncated traceback. Error messages are context-aware:
import errors suggest re-running setup.sh rather than checking SDR connections.

Increased stderr truncation limit from 200 to 500 chars and added full stderr
logging via logger.error() across all affected routes (radiosonde, ais, aprs,
acars, vdl2) for easier debugging.

Closes #173

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

routes/ais.py

@@ -450,9 +450,11 @@ def start_ais():
                     stderr_output = app_module.ais_process.stderr.read().decode('utf-8', errors='ignore').strip()
                 except Exception:
                     pass
+            if stderr_output:
+                logger.error(f"AIS-catcher stderr:\n{stderr_output}")
             error_msg = 'AIS-catcher failed to start. Check SDR device connection.'
             if stderr_output:
-                error_msg += f' Error: {stderr_output[:200]}'
+                error_msg += f' Error: {stderr_output[:500]}'
             return jsonify({'status': 'error', 'message': error_msg}), 500
 
         ais_running = True