feat: seed SSH users from UPTOP_ADMIN_KEY env and UPTOP_KEYS file

On startup, reads SSH public keys from two sources:
- UPTOP_ADMIN_KEY env var (single key, quick setup)
- UPTOP_KEYS file path (authorized_keys format, team setup)

Keys already in the database are skipped (idempotent across restarts).
All seeded users get admin role. Username parsed from key comment.

Dockerfile already sets UPTOP_KEYS=/data/authorized_keys — Docker users
just drop their keys file in ./data/ and start the container.

docker-compose.yml

@@ -14,5 +14,7 @@ services:
       - UPTOP_HTTP_PORT=8080
       - UPTOP_STATUS_ENABLED=true
       - UPTOP_STATUS_TITLE=System Status
+      # SSH access: add your public key via env var or authorized_keys file
+      # - UPTOP_ADMIN_KEY=ssh-ed25519 AAAA... you@host
     volumes:
       - ./data:/data