Refactor PostgreSQL provider: multi-schema & async prep

- Refactor migrations and provider to use multiple PostgreSQL schemas, each matching a legacy SQLite database (activitylog, authentication, displaypreferences, library, users).
- All tables, foreign keys, and indexes are now schema-qualified; Down migration drops tables by schema.
- Provider ensures schemas exist before migrations; entities are mapped to correct schemas in OnModelCreating.
- Add support for max-pool-size, min-pool-size, and multiplexing connection options; update logging accordingly.
- VACUUM ANALYZE now runs per schema during scheduled optimization.
- TruncateAllTablesAsync now truncates tables with schema qualification.
- README updated with schema structure, new options, and multiplexing warnings.
- CacheDecorator now calls async repository methods using .GetAwaiter().GetResult(), with documentation.
- Lays groundwork for full async/await and multiplexing support in the database layer.

src/Jellyfin.Database/Jellyfin.Database.Providers.Postgres/README.md

@@ -26,6 +26,8 @@ To use PostgreSQL as the database backend, configure the following options in yo
         { "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" }
       ]
@@ -83,19 +85,46 @@ psql -U jellyfin -h localhost jellyfin < jellyfin_backup.sql
 | 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
+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.