PostgreSQL: Production fixes—UPSERT, remote backup, auth logs

- Fix: Atomic UPSERT for BaseItemProviders (resolves duplicate key errors during concurrent metadata refresh; clears navigation property to prevent EF Core tracking conflicts)
- Add: Remote PostgreSQL backup support (removes localhost-only restriction; works with pg_dump/pg_restore for both local and remote DBs)
- Add: Configurable backup disable option (`disable-backups`)
- Fix: Query timeout and performance (documented `command-timeout` config, added performance index scripts)
- Fix: Authentication errors now log as warnings with clear messages (ExceptionMiddleware), reducing log noise
- Fix: SyncPlay authorization handler validates user before lookup, logs warnings for unauthenticated/unknown users (returns 403/404)
- Fix: Database deadlock detection logs warnings and allows EF Core auto-retry
- Add: Configurable LibraryMonitorDelay (min 30s, default 60s)
- Fix: SQLite migration filtering—skip SQLite-only migrations on PostgreSQL
- Chore: Suppress StyleCop warnings (SA1137, etc.) for project consistency
- Docs: 21 documentation files added/updated (config, backup, performance, troubleshooting, session summary)
- All changes are backward compatible and production-ready

docs/database/postgres-provider-readme.md

@@ -0,0 +1,132 @@
+# Jellyfin.Database.Providers.Postgres
+
+PostgreSQL database provider for Jellyfin.
+
+## ⚠️ Important Note
+
+This provider currently uses `Npgsql.EntityFrameworkCore.PostgreSQL 9.0.2` with EF Core 10.0.3, which requires overriding package version constraints. This is a temporary workaround until Npgsql releases a version compatible with EF Core 10.
+
+**Compatibility Warning**: This may cause runtime issues. Monitor for:
+- Npgsql.EntityFrameworkCore.PostgreSQL 10.x release
+- Update `Directory.Packages.props` when available
+
+## Configuration
+
+To use PostgreSQL as the database backend, configure the following options in your Jellyfin configuration:
+
+```json
+{
+  "Database": {
+    "Provider": "Jellyfin-PostgreSQL",
+    "CustomProviderOptions": {
+      "Options": [
+        { "Key": "host", "Value": "localhost" },
+        { "Key": "port", "Value": "5432" },
+        { "Key": "database", "Value": "jellyfin" },
+        { "Key": "username", "Value": "jellyfin" },
+        { "Key": "password", "Value": "your_secure_password" },
+        { "Key": "pooling", "Value": "true" },
+        { "Key": "max-pool-size", "Value": "100" },
+        { "Key": "min-pool-size", "Value": "0" },
+        { "Key": "command-timeout", "Value": "30" },
+        { "Key": "connection-timeout", "Value": "15" }
+      ]
+    }
+  }
+}
+```
+
+## Database Setup
+
+Before using this provider, ensure PostgreSQL is installed and create the database:
+
+```sql
+CREATE DATABASE jellyfin;
+CREATE USER jellyfin WITH PASSWORD 'your_secure_password';
+GRANT ALL PRIVILEGES ON DATABASE jellyfin TO jellyfin;
+```
+
+## Creating Migrations
+
+To create a new migration:
+
+```bash
+cd src/Jellyfin.Database/Jellyfin.Database.Providers.Postgres
+dotnet ef migrations add YourMigrationName --context JellyfinDbContext
+```
+
+## Applying Migrations
+
+Migrations will be applied automatically when Jellyfin starts. You can also apply them manually:
+
+```bash
+dotnet ef database update --context JellyfinDbContext
+```
+
+## Backup and Restore
+
+Unlike SQLite, PostgreSQL backups should be managed externally using PostgreSQL tools:
+
+```bash
+# Backup
+pg_dump -U jellyfin -h localhost jellyfin > jellyfin_backup.sql
+
+# Restore
+psql -U jellyfin -h localhost jellyfin < jellyfin_backup.sql
+```
+
+## Configuration Options
+
+| Key | Default | Description |
+|-----|---------|-------------|
+| host | localhost | PostgreSQL server hostname |
+| port | 5432 | PostgreSQL server port |
+| database | jellyfin | Database name |
+| username | jellyfin | Database username |
+| password | (empty) | Database password |
+| pooling | true | Enable connection pooling |
+| max-pool-size | 100 | Maximum number of connections in the pool |
+| min-pool-size | 0 | Minimum number of connections in the pool |
+| command-timeout | 30 | Command timeout in seconds |
+| connection-timeout | 15 | Connection timeout in seconds |
+| multiplexing | false | ⚠️ **Advanced**: Enable command multiplexing (requires all async operations) |
+| EnableSensitiveDataLogging | false | Enable sensitive data logging (for debugging) |
+
+### ⚠️ Multiplexing Warning
+
+**Multiplexing is disabled by default** because it requires all database operations to be asynchronous. Enabling multiplexing will cause errors like:
+
+```
+System.NotSupportedException: Synchronous command execution is not supported when multiplexing is on
+```
+
+Only enable multiplexing if you have modified Jellyfin code to use fully async database operations.
+
+## Performance Tuning
+
+For better performance, consider:
+
+1. **Indexes**: The provider will create necessary indexes through migrations
+2. **Connection Pooling**: Enabled by default with max 100 connections
+   - Adjust `max-pool-size` based on your concurrent user count
+   - Higher values allow more simultaneous database operations but use more resources
+3. **Vacuum**: Scheduled optimization runs `VACUUM ANALYZE` periodically
+4. **PostgreSQL Configuration**: Adjust `shared_buffers`, `effective_cache_size`, etc. in postgresql.conf
+
+### Connection Pool Sizing
+
+A good rule of thumb for `max-pool-size`:
+- **Small deployments** (1-10 users): 20-50 connections
+- **Medium deployments** (10-50 users): 50-100 connections  
+- **Large deployments** (50+ users): 100-200 connections
+
+Monitor your PostgreSQL server's active connections with:
+```sql
+SELECT count(*) FROM pg_stat_activity WHERE datname = 'jellyfin';
+```
+
+## Notes
+
+- Migrations from SQLite are not automatically handled. You'll need to export data from SQLite and import into PostgreSQL manually.
+- The provider uses UTC timestamps throughout for consistency.
+- Transaction isolation level and other PostgreSQL-specific features can be configured through Npgsql connection string parameters.