Add explanatory comments to 265 empty except blocks

CodeQL flags except blocks where `pass` has no comment explaining
why the exception is silently ignored (py/empty-except rule).

Added context-specific comments to all 265 instances across 31 files:
- database.py (~112): ALTER TABLE migrations — "Already applied"
- archive/library/3MF parsing (~64): "Skip unparseable metadata"
- virtual_printer network cleanup (~32): "Best-effort socket cleanup"
- discovery/SSDP (~13): "SO_REUSEPORT not available" / socket cleanup
- bambu_ftp/mqtt (~13): FTP cleanup, JSON decode, signal parsing
- remaining routes/services (~31): context-specific comments

backend/app/services/virtual_printer/ftp_server.py

@@ -118,14 +118,14 @@ class FTPSession:
             try:
                 await self.data_server.wait_closed()
             except OSError:
-                pass
+                pass  # Best-effort data server cleanup; may already be closed
             self.data_server = None
 
         try:
             self.writer.close()
             await self.writer.wait_closed()
         except OSError:
-            pass
+            pass  # Best-effort control connection cleanup; client may have disconnected
 
     # FTP Commands
 
@@ -303,7 +303,7 @@ class FTPSession:
                 self._data_writer.close()
                 await self._data_writer.wait_closed()
             except OSError:
-                pass
+                pass  # Best-effort data writer cleanup; peer may have closed already
             self._data_writer = None
             self._data_reader = None
 
@@ -312,7 +312,7 @@ class FTPSession:
                 self.data_server.close()
                 await self.data_server.wait_closed()
             except OSError:
-                pass
+                pass  # Best-effort data server shutdown; port may already be released
             self.data_server = None
 
         # Only delay if we actually closed something