feat: multi-actor ActivityPub support

Each registered user now has their own ActivityPub actor:
- Generate RSA keys per user on registration
- Webfinger resolves any registered user (@user@domain)
- Actor endpoints work for any registered user
- Each user has their own outbox (filtered activities)
- Activities signed with the publishing user's keys
- Objects attributed to the asset owner

Removed:
- ARTDAG_USER config (no longer single-actor)
- L1_SERVER config (comes with each request)

Added:
- /ui/users page listing all registered users
- user_exists() helper function

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.env.example

@@ -3,10 +3,9 @@
 # Domain for this ActivityPub server
 ARTDAG_DOMAIN=artdag.rose-ash.com
 
-# Default username (for actor endpoints)
-ARTDAG_USER=giles
-
 # JWT secret for token signing (generate with: openssl rand -hex 32)
 JWT_SECRET=your-secret-here-generate-with-openssl-rand-hex-32
 
-# Note: ARTDAG_L1 is no longer needed - L1 server URL is sent with each request
+# Notes:
+# - ARTDAG_USER removed - now multi-actor, each registered user is their own actor
+# - ARTDAG_L1 removed - L1 server URL is sent with each request