feat: add Letta server password protection for remote access
- Add SECURE and LETTA_SERVER_PASSWORD env vars to letta service
- Add letta.assistant.mosphere.at to Caddyfile.example
- Add LETTA_SERVER_PASSWORD to .env.example
When LETTA_SERVER_PASSWORD is set, Letta enables SECURE mode and
requires the password for API access. This allows connecting via
Letta Cloud ADE at https://app.letta.com → Self-Hosted Server.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>