Add plugin developer terms and conditions

[simonhamp]

Summary

• Adds formal Plugin Developer Terms and Conditions page at /developer-terms covering:
  • 30% platform fee on gross sales
  • Developer sole responsibility for plugin quality, performance, and customer support
  • NativePHP discretion over listing criteria, changes, and plugin removal
  • NativePHP discretion over plugin pricing and discounts
  • Payment terms, IP, data/privacy, termination, and standard legal boilerplate
• Requires terms acceptance during developer onboarding before Stripe Connect setup, with timestamp and version tracking on the developer_accounts table
• Adds "Third-Party Plugins" section to Terms of Service disclaiming liability for third-party plugins
• Adds "Third-Party Plugin Purchases" section to Privacy Policy explaining customer data sharing with plugin developers as a legitimate interest
• Adds "Developer Terms" link to the site footer
• Includes DeveloperAccountFactory with onboarded() and withAcceptedTerms() states

Test plan

• 14 feature tests covering:
  • Developer terms page accessibility and content
  • Onboarding requires terms checkbox (validation errors without it)
  • Terms acceptance timestamp and version are recorded
  • Existing terms acceptance is not overwritten on re-onboarding
  • Model hasAcceptedPluginTerms() and hasAcceptedCurrentTerms() methods
  • ToS and Privacy Policy contain new sections
  • Onboarding view shows checkbox vs. accepted confirmation appropriately
• Manual review of legal content by a lawyer before merging

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