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/core/config.py

@@ -84,6 +84,7 @@ class Settings(BaseSettings):
     RESEND_API_KEY: Optional[str] = None
     FROM_EMAIL: str = "ResolutionFlow <invites@resolutionflow.com>"
     FEEDBACK_EMAIL: Optional[str] = None
+    SALES_LEAD_RECIPIENT_EMAIL: str = "sales@resolutionflow.com"
 
     @property
     def email_enabled(self) -> bool: