feat: add fluxer upstream source and self-hosting documentation

- Clone of github.com/fluxerapp/fluxer (official upstream)
- SELF_HOSTING.md: full VM rebuild procedure, architecture overview,
  service reference, step-by-step setup, troubleshooting, seattle reference
- dev/.env.example: all env vars with secrets redacted and generation instructions
- dev/livekit.yaml: LiveKit config template with placeholder keys
- fluxer-seattle/: existing seattle deployment setup scripts

fluxer/.github/pull_request_template.md

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+## Summary
+
+<!-- A few bullets is perfect: what changed, why it changed, and anything reviewers should pay attention to. -->
+
+- **What:**
+- **Why:**
+- **Notes for reviewers:**
+
+## How to verify
+
+<!-- Concrete steps to validate the change. Include screenshots/recordings for UI changes when helpful. -->
+
+1.
+2.
+3.
+
+## Tests
+
+<!-- List what you ran, or explain why tests weren't added/changed. -->
+
+- [ ] Added/updated unit tests (where it makes sense)
+- [ ] Manual verification:
+
+## Checklist
+
+- [ ] PR targets `canary`
+- [ ] PR title follows Conventional Commits (mostly lowercase)
+- [ ] CI is green (or I'm actively addressing failures)
+
+## Screenshots / recordings (UI changes)
+
+<!-- Drag and drop images/videos here. -->