feat(sales): add POST /sales-leads public endpoint

Phase 2 Task 29 — public Talk-to-Sales submission endpoint.

- New POST /api/v1/sales-leads (public, no auth, rate-limited 5/hour per IP).
- Inserts a sales_leads row, fires best-effort notification email and
  PostHog server-side capture; failures are logged but never fail the
  request.
- New EmailService.send_sales_lead_notification static method.
- New SALES_LEAD_RECIPIENT_EMAIL setting (defaults to sales@resolutionflow.com).
- Schemas: SalesLeadCreate / SalesLeadCreateResponse with literal source enum.
- Tests: happy path (row + email), email-failure resilience, and rate-limit
  enforcement (re-enables the slowapi limiter for the rate-limit assertion
  since DEBUG=true disables it by default in tests).

PostHog server-side instrumentation point is wired in but no-ops gracefully
until app.core.analytics.posthog exists — turning it on is a one-line
change when the backend SDK is configured.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/api/router.py

@@ -26,6 +26,7 @@ from app.api.endpoints import (
     billing,
     beta_feedback,
     beta_signup,
+    sales_leads,
     branding,
     categories,
     copilot,
@@ -88,6 +89,7 @@ api_router.include_router(billing.router)     # Reachable when subscription lock
 api_router.include_router(shared.router)       # Public share links (no auth)
 api_router.include_router(shares.public_router)  # Public session share links (optional auth)
 api_router.include_router(beta_signup.router)
+api_router.include_router(sales_leads.router)  # Talk-to-Sales (no auth, rate-limited)
 api_router.include_router(webhooks.router)     # Stripe webhook receiver
 api_router.include_router(public_templates.router)  # Public gallery (no auth, rate-limited)
 api_router.include_router(survey.router)       # Public survey flow (no auth, rate-limited)