feat(pricing): add /pricing page (B-style)

Phase 2 Task 42: public pricing page gated by SELF_SERVE_ENABLED.

Backend:
- New `GET /api/v1/plans/public` (no auth) returns plan_billing rows
  joined with plan_limits.max_users (as `max_seats`), filtered to
  is_public=true AND is_archived=false, ordered by sort_order ASC,
  plan ASC. Uses get_admin_db (cross-tenant catalog read, same pattern
  as /config/public).
- `PublicPlanResponse` schema in app/schemas/billing.py.
- Registered as PUBLIC in api router.

Frontend:
- `plansApi.getPublic()` client (frontend/src/api/plans.ts).
- `PricingPage` at /pricing with hero / 3 plan cards (Pro recommended,
  Enterprise hides price) / hardcoded v1 comparison table / testimonial
  placeholder / soft trust strip.
- Reads `useAppConfig().self_serve_enabled`; renders a 404 fallback
  when disabled, never calls the API in that path.
- Start free trial CTAs link to /register?plan=starter|pro; Talk to sales
  links to /contact-sales (page wired in Task 43).

Tests:
- Backend: only-public-rows + sort-order ordering.
- Frontend (Vitest): three plan cards with API prices, /register?plan=pro
  CTA, /contact-sales CTA, 404 when self_serve_enabled is false, soft
  trust language (no SOC2 claim).

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

backend/app/schemas/billing.py

@@ -42,3 +42,23 @@ class BillingStateResponse(BaseModel):
     plan_billing: Optional[PlanBillingState]
     plan_limits: Dict[str, Any]
     enabled_features: Dict[str, bool]
+
+
+class PublicPlanResponse(BaseModel):
+    """Public-safe view of a billable plan, used by the marketing /pricing page.
+
+    Sourced from `plan_billing` joined with `plan_limits.max_users` (exposed
+    here as `max_seats`). Always filtered server-side to is_public=True and
+    is_archived=False, so `is_public` is a constant True for any row returned
+    here — included for clarity and forward compatibility.
+    """
+    plan: str
+    display_name: str
+    description: Optional[str] = None
+    monthly_price_cents: Optional[int] = None
+    annual_price_cents: Optional[int] = None
+    max_seats: Optional[int] = None
+    sort_order: int
+    is_public: bool = True
+
+    model_config = {"from_attributes": True}