feat: 5-day theme-level onboarding track from the "New here?" card (#30)

A self-paced onboarding track that introduces a new employee to every KB
theme in breadth (not depth), so they grasp how Respellion works day to
day and week to week. Offered as a CTA inside the Dashboard "New here?"
explainer card; always available regardless of enrollment.

Design:
- Theme is the trackable unit; the 5 "days" are a read-time presentation
  grouping, so re-chunking never loses progress. Completion is stored per
  theme in onboarding_completions.
- Per-theme overview generated lazily on first open (fast-tier
  emit_onboarding_overview tool), cached in onboarding_overviews keyed by
  theme + a topics_fingerprint that triggers regeneration when the theme's
  topic set changes.
- Reachable via /onboarding-track using the existing skipEnrollmentGate
  prop, decoupled from the 26-week curriculum (distinct from /onboarding,
  the enrollment page).

Backend:
- pb_migrations/1781200000_created_onboarding.js: two collections with
  authenticated-only rules and unique indexes; TEXT team_member_id (no
  relation) per the post-#18/#27 convention. Mirrored in
  scripts/setup-pb-collections.mjs.
- src/lib/onboardingService.js: pure helpers (orderThemes,
  distributeThemesIntoDays, computeTopicsFingerprint,
  computeOnboardingProgress, buildOnboardingPlan) + generation + I/O.
- db.js onboarding helpers use pb.filter() bindings (theme is free text).
- LLM tool + Zod schema + registry + simulation stub.

Frontend:
- src/pages/OnboardingTrack.jsx (day list, per-theme overview, completion
  banner, progress ring/day bar).
- Dashboard "New here?" card CTA + X/5-days progress chip (hidden when the
  KB has no themes).

Docs: data-model, generation-spec (§D), frontend-spec updated.

Verified: 22 new unit tests (npm test 134/134), eslint clean on changed
files, npm run build OK, PocketBase v0.30.4 boot applies the migration
(collections + unique indexes + authed rules confirmed), and a backend
contract check (upsert idempotency, unique-index guard, special-char
theme filtering).

Closes #30

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

docs/generation-spec.md

@@ -74,6 +74,29 @@ explanation, difficulty }`.
 
 ---
 
+## D. Onboarding overviews — `src/lib/onboardingService.js`
+
+Powers the 5-day onboarding track (issue #30): a short, breadth-first overview of
+**one theme** for a brand-new employee — deliberately light, not a deep lesson.
+
+- **Tool:** `emit_onboarding_overview` · **tier:** `fast` (Haiku) · `maxTokens: 1500`,
+  60s timeout · schema `onboardingOverviewSchema`.
+- **Shape:** `{ title, what_it_is, why_it_matters, key_points[3–5], topics_covered[{topic_id,label}] }`.
+  `why_it_matters` is framed around day-to-day / week-to-week work at Respellion.
+- **Cache:** `onboarding_overviews`, one row per theme, keyed by theme name plus a
+  `topics_fingerprint` (stable hash of the theme's sorted topic ids).
+  `getOrGenerateOnboardingOverview(theme, topics, {force})` returns the cached row when
+  the fingerprint matches; a mismatch (topic added/removed) or `force` regenerates.
+- **Simulation:** `emit_onboarding_overview` has a stub in `SIMULATION_TOOL_STUBS`.
+
+Theme ordering + day grouping are pure helpers in the same module
+(`orderThemes`, `distributeThemesIntoDays`, `computeTopicsFingerprint`,
+`computeOnboardingProgress`, `buildOnboardingPlan`), unit-tested in
+`src/lib/__tests__/onboardingService.test.js`. Completion is recorded per **theme**
+in `onboarding_completions` (`{ team_member_id, theme }`), not per day.
+
+---
+
 ## Shared infrastructure (`src/lib/llm.js`)
 
 - **Tiers:** `fast` (Haiku 4.5), `standard` (Sonnet 4.6), `reasoning` (Opus 4.7);