fix(blobs): increase thumbnail size limit from 1MB to 6MB

Aggregator thumbnail uploads were failing for larger images. Increased
the blob size limit to 6MB to match Bluesky's standard blob limits.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <[e-mail]>

internal/atproto/lexicon/social/coves/embed/external.json

···
         "thumb": {
           "type": "blob",
           "accept": ["image/png", "image/jpeg", "image/webp"],
-          "maxSize": 1000000,
+          "maxSize": 6000000,
           "description": "Thumbnail image for the post (applies to primary link)"
         },
         "domain": {

internal/core/blobs/service.go

···
 		return nil, fmt.Errorf("failed to read image data: %w", err)
 	}
 
-	// Validate size (1MB = 1048576 bytes)
-	const maxSize = 1048576
+	// Validate size (6MB = 6291456 bytes)
+	const maxSize = 6291456
 	if len(data) > maxSize {
-		return nil, fmt.Errorf("image size %d bytes exceeds maximum of %d bytes (1MB)", len(data), maxSize)
+		return nil, fmt.Errorf("image size %d bytes exceeds maximum of %d bytes (6MB)", len(data), maxSize)
 	}
 
 	// Upload to PDS
···
 		return nil, fmt.Errorf("unsupported MIME type: %s (allowed: image/jpeg, image/png, image/webp)", mimeType)
 	}
 
-	// Validate size (1MB = 1048576 bytes)
-	const maxSize = 1048576
+	// Validate size (6MB = 6291456 bytes)
+	const maxSize = 6291456
 	if len(data) > maxSize {
-		return nil, fmt.Errorf("data size %d bytes exceeds maximum of %d bytes (1MB)", len(data), maxSize)
+		return nil, fmt.Errorf("data size %d bytes exceeds maximum of %d bytes (6MB)", len(data), maxSize)
 	}
 
 	// Use community's PDS URL (for federated communities)