fix(web): say what actually went wrong instead of "unknown"

Every failing endpoint returned "An error occurred; see logs for
details" and nothing else. That is survivable until the logs are the
thing you cannot reach: a device whose SD card was failing answered the
restart action, /system/status and /logs with that same sentence -- the
log viewer included, because journalctl could not be executed -- while
the exception underneath said

    [Errno 5] Input/output error: 'systemctl'

which names the fault outright. The only endpoint that helped was
/health, and only because it happens to pass a subprocess's stderr
through. Diagnosis came down to guessing which endpoint leaked something.

Add describe_exception(), returning "TypeName: message" on one line, and
populate the `details` field that the response schema has always had and
nothing ever filled. The type alone carries information -- a bare
PermissionError says more than any generic sentence.

Exception text is not automatically safe to echo: a requests error
quotes the URL it failed on, and plugins that authenticate by query
string put their key there. Credential values are redacted while the
parameter name is kept, since knowing which credential was involved is
part of the diagnosis. Length is capped and newlines collapsed so a
parser's context cannot flood a JSON field.

Nine handlers in api_v3 bound the exception and never used it, so the
promised log entry was never written either -- "see logs for details"
was false, not merely unhelpful. Those now log with a traceback and
carry the detail. The other 60 already logged and are unchanged; they
can adopt the helper as they are touched.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Udr6MfaFLUPhX5Fgo67Jf5

web_interface/blueprints/api_v3.py

@@ -21,6 +21,7 @@ logger = logging.getLogger(__name__)
 # Import new infrastructure
 from src.web_interface.api_helpers import success_response, error_response, validate_request_json
 from src.web_interface.errors import ErrorCode
+from src.web_interface.error_handler import describe_exception
 from src.plugin_system.operation_types import OperationType
 from src.web_interface.validators import (
     validate_file_upload
@@ -289,9 +290,11 @@ def get_schedule_config():
 
         return success_response(data=schedule_config)
     except Exception as e:
+        logger.error("%s failed", request.path, exc_info=True)
         return error_response(
             ErrorCode.CONFIG_LOAD_FAILED,
             "An error occurred; see logs for details",
+            details=describe_exception(e),
             status_code=500
         )
 
@@ -1625,9 +1628,11 @@ def get_health():
 
         return jsonify({'status': 'success', 'data': health_status})
     except Exception as e:
+        logger.error("%s failed", request.path, exc_info=True)
         return jsonify({
             'status': 'error',
             'message': 'An error occurred; see logs for details',
+            'details': describe_exception(e),
             'data': {'status': 'unhealthy'}
         }), 500
 
@@ -6563,7 +6568,10 @@ def get_fonts_catalog():
 
         return jsonify({'status': 'success', 'data': {'catalog': catalog}})
     except Exception as e:
-        return jsonify({'status': 'error', 'message': 'An error occurred; see logs for details'}), 500
+        logger.error("%s failed", request.path, exc_info=True)
+        return jsonify({'status': 'error',
+                        'message': 'An error occurred; see logs for details',
+                        'details': describe_exception(e)}), 500
 
 @api_v3.route('/fonts/tokens', methods=['GET'])
 def get_font_tokens():
@@ -7522,9 +7530,11 @@ def get_logs():
             'message': 'Timeout while fetching logs'
         }), 500
     except Exception as e:
+        logger.error("%s failed", request.path, exc_info=True)
         return jsonify({
             'status': 'error',
-            'message': 'An error occurred; see logs for details'
+            'message': 'An error occurred; see logs for details',
+            'details': describe_exception(e)
         }), 500
 
 # Multi-Display Sync Endpoints
@@ -7589,9 +7599,11 @@ def get_wifi_status():
             }
         })
     except Exception as e:
+        logger.error("%s failed", request.path, exc_info=True)
         return jsonify({
             'status': 'error',
-            'message': 'An error occurred; see logs for details'
+            'message': 'An error occurred; see logs for details',
+            'details': describe_exception(e)
         }), 500
 
 @api_v3.route('/wifi/scan', methods=['GET'])
@@ -7764,9 +7776,11 @@ def enable_ap_mode():
                 'message': message
             }), 400
     except Exception as e:
+        logger.error("%s failed", request.path, exc_info=True)
         return jsonify({
             'status': 'error',
-            'message': 'An error occurred; see logs for details'
+            'message': 'An error occurred; see logs for details',
+            'details': describe_exception(e)
         }), 500
 
 @api_v3.route('/wifi/ap/disable', methods=['POST'])
@@ -7789,9 +7803,11 @@ def disable_ap_mode():
                 'message': message
             }), 400
     except Exception as e:
+        logger.error("%s failed", request.path, exc_info=True)
         return jsonify({
             'status': 'error',
-            'message': 'An error occurred; see logs for details'
+            'message': 'An error occurred; see logs for details',
+            'details': describe_exception(e)
         }), 500
 
 @api_v3.route('/wifi/ap/auto-enable', methods=['GET'])
@@ -7810,9 +7826,11 @@ def get_auto_enable_ap_mode():
             }
         })
     except Exception as e:
+        logger.error("%s failed", request.path, exc_info=True)
         return jsonify({
             'status': 'error',
-            'message': 'An error occurred; see logs for details'
+            'message': 'An error occurred; see logs for details',
+            'details': describe_exception(e)
         }), 500
 
 @api_v3.route('/wifi/ap/auto-enable', methods=['POST'])
@@ -7842,9 +7860,11 @@ def set_auto_enable_ap_mode():
             }
         })
     except Exception as e:
+        logger.error("%s failed", request.path, exc_info=True)
         return jsonify({
             'status': 'error',
-            'message': 'An error occurred; see logs for details'
+            'message': 'An error occurred; see logs for details',
+            'details': describe_exception(e)
         }), 500
 
 @api_v3.route('/wifi/radio', methods=['GET'])