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.

Jellyfin.Server.Implementations/Item/MediaAttachmentRepository.cs

@@ -9,6 +9,7 @@ using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
 using System.Threading;
+using System.Threading.Tasks;
 using Jellyfin.Database.Implementations;
 using Jellyfin.Database.Implementations.Entities;
 using MediaBrowser.Controller.Persistence;
@@ -22,38 +23,53 @@ using Microsoft.EntityFrameworkCore;
 public class MediaAttachmentRepository(IDbContextFactory<JellyfinDbContext> dbProvider) : IMediaAttachmentRepository
 {
     /// <inheritdoc />
-    public void SaveMediaAttachments(
+    public async Task SaveMediaAttachmentsAsync(
         Guid id,
         IReadOnlyList<MediaAttachment> attachments,
-        CancellationToken cancellationToken)
+        CancellationToken cancellationToken = default)
     {
-        using var context = dbProvider.CreateDbContext();
-        using var transaction = context.Database.BeginTransaction();
+        await using var context = dbProvider.CreateDbContext();
+        await using var transaction = await context.Database.BeginTransactionAsync(cancellationToken).ConfigureAwait(false);
 
         // Users may replace a media with a version that includes attachments to one without them.
         // So when saving attachments is triggered by a library scan, we always unconditionally
         // clear the old ones, and then add the new ones if given.
-        context.AttachmentStreamInfos.Where(e => e.ItemId.Equals(id)).ExecuteDelete();
+        await context.AttachmentStreamInfos
+            .Where(e => e.ItemId.Equals(id))
+            .ExecuteDeleteAsync(cancellationToken)
+            .ConfigureAwait(false);
+
         if (attachments.Any())
         {
-            context.AttachmentStreamInfos.AddRange(attachments.Select(e => Map(e, id)));
+            await context.AttachmentStreamInfos
+                .AddRangeAsync(attachments.Select(e => Map(e, id)), cancellationToken)
+                .ConfigureAwait(false);
         }
 
-        context.SaveChanges();
-        transaction.Commit();
+        await context.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
+        await transaction.CommitAsync(cancellationToken).ConfigureAwait(false);
     }
 
     /// <inheritdoc />
-    public IReadOnlyList<MediaAttachment> GetMediaAttachments(MediaAttachmentQuery filter)
+    public async Task<IReadOnlyList<MediaAttachment>> GetMediaAttachmentsAsync(
+        MediaAttachmentQuery filter,
+        CancellationToken cancellationToken = default)
     {
-        using var context = dbProvider.CreateDbContext();
-        var query = context.AttachmentStreamInfos.AsNoTracking().Where(e => e.ItemId.Equals(filter.ItemId));
+        await using var context = dbProvider.CreateDbContext();
+        var query = context.AttachmentStreamInfos
+            .AsNoTracking()
+            .Where(e => e.ItemId.Equals(filter.ItemId));
+
         if (filter.Index.HasValue)
         {
             query = query.Where(e => e.Index == filter.Index);
         }
 
-        return query.AsEnumerable().Select(Map).ToArray();
+        var attachments = await query
+            .ToArrayAsync(cancellationToken)
+            .ConfigureAwait(false);
+
+        return attachments.Select(Map).ToArray();
     }
 
     private MediaAttachment Map(AttachmentStreamInfo attachment)