feat: enforce default channel validation (max 3, one per platform)

[Copilot]

Summary

Implements backend validation to prevent public channel conflicts: maximum 3 public channels per app, and only one public channel per platform (iOS/Android/Electron). Users can now have either one universal default channel OR up to three platform-specific channels without ambiguity.

Implementation:

• Validation logic (utils/supabase.ts): validatePublicChannels() runs pre-upsert, checks channel count and platform uniqueness, throws typed errors (max_public_channels, duplicate_platform_{ios|android|electron})
• Channel API (public/channel/post.ts): Added electron field support to match iOS/Android pattern
• Test coverage (channel_default_validation.test.ts): Valid configs (single multi-platform, three single-platform, mixed), invalid configs (4+ channels, platform duplicates), update scenarios
• Documentation (docs/DEFAULT_CHANNEL_VALIDATION.md): Rules reference, error messages, migration guidance

Example validation:

// ✓ Valid: One universal channel
{ public: true, ios: true, android: true, electron: true }

// ✓ Valid: Three platform-specific
[
  { public: true, ios: true, android: false, electron: false },
  { public: true, ios: false, android: true, electron: false },
  { public: true, ios: false, android: false, electron: true }
]

// ✗ Invalid: Duplicate iOS in public channels
[
  { public: true, ios: true, ... },
  { public: true, ios: true, ... }  // throws duplicate_platform_ios
]

Private channels unaffected. Validation uses final channel state (merges update with existing values).

Test plan

1. Run bun test:backend to verify validation test suite
2. Create app with 3 public channels (different platforms) → succeeds
3. Attempt 4th public channel → receives max_public_channels error
4. Create 2 public channels with same platform → receives duplicate_platform_* error
5. Update existing public channel to conflict → validation prevents

Screenshots

N/A - Backend validation only

Checklist

• My code follows the code style of this project and passes bun run lint:backend && bun run lint.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• My change has adequate E2E test coverage.
• I have tested my code manually, and I have provided steps how to reproduce my tests

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[coderabbitai]

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[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

Warning

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I tried to connect to the following addresses, but was blocked by firewall rules:

• 127.0.0.11
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• https://api.github.com/repos/supabase/cli/releases/latest
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[sonarqubecloud]

Quality Gate passed

Issues
8 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