Platform config templates & Postgres migration recovery

- Added Linux/Windows startup config templates and README
- Updated default paths to FHS-compliant locations
- Improved publish profiles for platform targeting
- Enhanced Postgres migration: auto-recover on 42P07 errors
- Added RecordMigrationAsAppliedAsync for manual migration history
- Improved migration logging and error handling
- Added detailed documentation for config and migration fixes

Jellyfin.Server/Resources/Configuration/startup.linux.json

@@ -0,0 +1,14 @@
+{
+  "$schema": "https://json.schemastore.org/jellyfin-startup",
+  "// Comment": "Linux-specific startup configuration for Jellyfin Server",
+  "// Instructions": "Copy this file to 'startup.json' in the Jellyfin directory",
+  "// Documentation": "These paths follow Linux FHS (Filesystem Hierarchy Standard)",
+  "Paths": {
+    "DataDir": "/var/lib/jellyfin",
+    "ConfigDir": "/var/lib/jellyfin",
+    "CacheDir": "/var/lib/jellyfin",
+    "LogDir": "/var/lib/jellyfin",
+    "TempDir": "/var/lib/jellyfin",
+    "WebDir": "/var/lib/jellyfin"
+  }
+}