docs: realign blueprint with shipped state + add feature/journey/ideas docs

- PROJECT.md, README.md, TEST_GUIDE.md: status line refreshed; rate-limiter
  doc-vs-code drift fixed; HTML export section rewritten for the SvelteKit-
  static viewer; SSE event names + new events documented; config seed block
  extended with planned toggles + privacy_note; decision log entries added.
- docs/CONCEPT_HTML_VIEWER.md, docs/CONCEPT_MOBILE_UI.md: banner the design
  intent as shipped; point at the source-of-truth code paths.
- docs/CONCEPT_DIASHOW.md: planned-then-shipped design for the live diashow
  (two-queue policy, pluggable transitions, data-mode aware).
- docs/FEATURES.md: capability matrix by role (Guest / Host / Admin) plus
  prose per area (auth, posting, feed, moderation, admin, export, gestures,
  data mode, quotas, privacy note, extensibility).
- docs/USER_JOURNEYS.md: step-by-step flows for every supported scenario,
  including PIN reset by host, data mode, privacy note, gestures, and the
  admin toggles.
- docs/IDEAS.md: speculative extensions (global diashow, reactions,
  multi-tenancy, animation pack, etc.) — explicitly out of v0.16 scope.
- backend/migrations/README.md, frontend/src/lib/README.md: codify the
  "never edit a shipped migration" rule and the lib/ conventions
  (one store per concern, gestures via actions, sheets via ContextSheet,
  transitions as drop-in components).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/migrations/README.md

@@ -0,0 +1,28 @@
+# Migrations
+
+SQLx-managed Postgres migrations. Each `NNN_topic.up.sql` has a matching
+`NNN_topic.down.sql`. Run by `sqlx::migrate!()` at app start.
+
+## Rules
+
+1. **Never edit a shipped migration.** If a column needs to change or a fix needs to
+   land, write a new migration. Production has already applied the old one and SQLx
+   tracks each by checksum — editing in place will fail to apply on existing databases.
+2. **Always pair `.up.sql` with a `.down.sql`.** Reverts may not be perfect (data
+   loss is sometimes unavoidable) but the file must exist and do the best it can.
+3. **Prefer additive changes.** New columns, new tables, new keys in `config`. Drop /
+   rename only when there is no alternative.
+4. **No business logic in migrations.** Schema + seeds only. Anything that needs Rust
+   code goes in a one-off binary, not a migration file.
+5. **One concern per migration.** Easier to revert. Easier to read in `git log`.
+
+## Numbering
+
+Zero-padded three digits, monotonically increasing. The next free number lives at the
+bottom of the directory listing — pick that.
+
+## Seed-only migrations
+
+When you only need to add `config` keys (feature flags, defaults), use
+`INSERT … ON CONFLICT DO NOTHING` so existing operator overrides survive. See
+`009_feature_toggles.up.sql` for the canonical shape.