Minimize avoidable chat and signaling metadata disclosure

[Copilot]

Reduce plaintext metadata exposed by chat and signaling without breaking existing payload contracts. Document what remains visible despite E2EE.

• Count-only presence: Add getParticipantCount() and countOnly=true; migrate UI presence checks and SDK call preconditions. Preserve the legacy identity-list API and accept older servers’ list responses.
• Payload and log minimization: Explicitly select join fields and envelope headers (version, strategy, data), preserving opaque strategy data. Remove identifiers from join/channel diagnostics.
• Contract coverage: Add exact-field assertions for socket payloads, relay acknowledgments, count-only responses, legacy compatibility, and identifier-free logging.
• Privacy documentation: Record the before/after inventory, remaining routing and traffic-pattern exposure, ephemeral-ID guidance, and optional authenticated-padding trade-offs.

const count = await chat.getParticipantCount(); // No participant IDs requested

• Fixes Audit and minimize avoidable metadata leakage #496

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[Copilot]

Copilot review overview

🔵 Needs a closer look

A moderate privacy issue remains because relay-visible IDs use Math.random() instead of a CSPRNG.

Review effort: Lite
Findings: None

What changed in this PR

This PR reduces chat and signaling metadata exposure while preserving legacy APIs and documenting remaining E2EE metadata visibility.

Changes:

• Adds count-only participant presence with legacy compatibility.
• Minimizes socket payloads and identifier-bearing diagnostics.
• Adds contract tests and privacy documentation.

File	Description
service/​src/​socket/​socket.ts	Minimizes socket payload fields.
service/​src/​socket/​socket.test.ts	Tests payload and logging contracts.
service/​src/​sdk.ts	Adds count-based presence checks.
service/​src/​sdk.test.ts	Tests count usage and privacy logging.
service/​src/​public/​types.ts	Exposes participant-count API.
service/​src/​api/​messages.ts	Implements count-only requests and fallback.
service/​src/​api/​messages.test.ts	Tests count and compatibility behavior.
service/​README.md	Documents SDK privacy behavior.
README.md	Documents metadata limitations.
client/​src/​context/​ChatContext.tsx	Uses count-only presence checks.
backend/​socket.io/​listeners.ts	Removes room identifiers from diagnostics.
backend/​socket.io/​listeners.test.ts	Tests relay and logging contracts.
backend/​README.md	Adds metadata inventory and privacy guidance.
backend/​api/​messaging/​index.ts	Supports count-only responses.
backend/​api/​messaging/​index.test.ts	Tests count and compatibility behavior.

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