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/data-model.md

@@ -212,6 +212,34 @@ Best-effort telemetry for every Anthropic call (written by `callLLM`).
 
 ---
 
+### `onboarding_overviews`
+Cached, breadth-first per-theme overview for the onboarding track (issue #30).
+One row per theme, shared across all users. Generated lazily on first open.
+
+| Field | Type | Notes |
+|---|---|---|
+| theme | text | canonical theme name (from `topics.theme`); **unique** |
+| content | json | validated `onboardingOverviewSchema` payload (title, what_it_is, why_it_matters, key_points, topics_covered) |
+| topics_fingerprint | text | stable hash of the theme's sorted topic ids; a mismatch triggers regeneration |
+
+Unique index on `(theme)`. Rules: authenticated-only (`@request.auth.id != ""`).
+
+---
+
+### `onboarding_completions`
+Per-user, per-theme completion marker for the onboarding track (issue #30).
+
+| Field | Type | Notes |
+|---|---|---|
+| team_member_id | text | the employee — **plain text**, not a relation (admins can delete members; orphan rows are harmless) |
+| theme | text | the theme marked complete |
+
+Unique index on `(team_member_id, theme)` → idempotent. Rules: authenticated-only.
+Completion is tracked per **theme**, not per day; the 5 "days" are a read-time
+presentation grouping, so re-chunking never loses progress.
+
+---
+
 ## Dropped / legacy collections
 
 These existed in earlier iterations and have been removed. Their `db.js` helpers