feat(sales): add /contact-sales form + landing page CTA

Public Talk-to-Sales surface and a "See pricing" hero CTA on the marketing
landing page. Phase 2 Task 43 of self-serve signup.

- frontend/src/api/sales.ts: salesApi.createLead -> POST /sales-leads.
- ContactSalesPage at /contact-sales (public, gated by self_serve_enabled
  with a 404-style fallback). Form fields: name, work email, company,
  team size (1-2 / 3-5 / 6-10 / 11-25 / 26+), and an optional
  "what brought you here?" textarea -> message. Submit button disabled
  while in flight to block duplicate submissions.
- Confirmation surface replaces the form on success. Calendly block is
  hidden when VITE_CALENDLY_URL is unset.
- detectSource(): 'pricing_page' if document.referrer contains '/pricing',
  else 'landing_page'. Server emits the canonical PostHog
  talk_to_sales_form_submitted event with this source.
- LandingPage: new "See pricing" hero CTA gated by useAppConfig().
  self_serve_enabled.
- frontend/.env.example + Dockerfile: VITE_CALENDLY_URL ARG/ENV.
- Tests: ContactSalesPage submit/confirmation, Calendly hide-when-unset,
  in-flight de-dup, 404 when self-serve off; LandingPage CTA on/off.

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

frontend/src/api/sales.ts

@@ -0,0 +1,32 @@
+import apiClient from './client'
+
+export type SalesLeadSource = 'pricing_page' | 'register_footer' | 'landing_page'
+
+export interface SalesLeadCreatePayload {
+  email: string
+  name: string
+  company: string
+  team_size?: string
+  message?: string
+  source: SalesLeadSource
+  posthog_distinct_id?: string
+}
+
+export interface SalesLeadCreateResponse {
+  id: string
+  status: 'received'
+}
+
+export const salesApi = {
+  /**
+   * Public Talk-to-Sales submission. No auth required. Rate-limited per IP
+   * server-side (5/hour). Server emits PostHog `talk_to_sales_form_submitted`
+   * — frontend should NOT also fire this event.
+   */
+  async createLead(payload: SalesLeadCreatePayload): Promise<SalesLeadCreateResponse> {
+    const response = await apiClient.post<SalesLeadCreateResponse>('/sales-leads', payload)
+    return response.data
+  },
+}
+
+export default salesApi