- Store avatars in 3-level deep hash bucket directories based on
SHA256 of DID to prevent large flat directories
- Cache thumbnails alongside full-size images to avoid repeated scaling
- Return placeholder avatar for invalid/unresolvable identifiers
instead of 404 errors
- Support did:web identifiers (already handled by Indigo library)

Replace simple "OK" response with embedded ASCII art banner text file
on the "/" endpoint.

- Main endpoint /{identifier} now returns avatars scaled to max 1000x1000
(preserving aspect ratio, unchanged if smaller)
- New /thumbnail/{identifier} endpoint returns avatars scaled to 128x128