Add planning docs for self-hosted cycling app

Planning output only; no application code yet.

Key findings driving the design:

- The Bryton Rider 650 has on-device Wi-Fi (Main Menu -> Data Sync) and
  uploads to Bryton's cloud with no phone and no Bryton Active app. Paired
  with the reverse-engineered Bryton cloud API — which returns the original
  unmodified FIT bytes — this makes ride sync fully hands-off, and higher
  fidelity than the current Strava route (Strava's API cannot return the
  original file, only smoothed streams).
- Build fresh rather than forking Endurain or FitTrackee; borrow Endurain's
  gear/component structure and strava-gear's retroactive time-ranged wear
  computation.
- PWA rather than a native iOS app: iOS 16.4+ gives home-screen PWAs real
  push notifications, which was the only thing that used to force native.

Docs:
  docs/PLAN.md       stack, schema, ingestion, auth, notifications, roadmap,
                     CI/CD, risks, verification
  docs/RESEARCH.md   Bryton cloud protocol, FIT library comparison,
                     maintenance intervals, geo services, Gitea gotchas
  docs/DECISIONS.md  decisions taken, alternatives rejected, rationale

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,39 @@
+# Python
+__pycache__/
+*.py[cod]
+.venv/
+venv/
+.mypy_cache/
+.ruff_cache/
+.pytest_cache/
+*.egg-info/
+
+# Node
+node_modules/
+.svelte-kit/
+build/
+dist/
+.vite/
+
+# Environment & secrets
+.env
+.env.*
+!.env.example
+*.pem
+*.key
+
+# Data — never commit ride files, blobs, or dumps
+blobstore/
+import_inbox/
+*.fit
+*.gpx
+*.tcx
+*.sql
+*.dump
+!apps/api/tests/fixtures/fit/*.fit
+
+# OS / editor
+.DS_Store
+.idea/
+.vscode/
+*.swp