Audit and minimize avoidable metadata leakage

[muke1908]

Split out from #403 follow-up planning.

What to do

• Audit all chat/signaling payloads for plaintext fields visible to server/network observers.
• Identify avoidable metadata (user-identifying fields, verbose event types, predictable room identifiers, etc.).
• Propose and implement low-risk reductions (field minimization, opaque IDs where possible, optional padding strategy discussion).
• Add tests to ensure message/signaling payload contracts stay minimal.
• Document remaining unavoidable metadata and why it is unavoidable.

Learning opportunities

• Metadata security vs content security in E2EE systems.
• Practical limits of hiding traffic patterns in browser-based real-time apps.
• Designing schemas with privacy-by-default.

Acceptance criteria

• A clear before/after metadata inventory exists.
• Avoidable plaintext metadata is reduced without breaking compatibility.
• Tests and docs capture the intended minimal surface.