[Feature] Cost Tracking & API Key Management - Monitor Spending Across All AI Services

[vzeman]

Summary

Implement secure API key management and comprehensive cost tracking across all integrated AI services (Replicate, ElevenLabs, Anthropic Claude). Visibility into spending is essential for users who rely on pay-per-use APIs.

Requirements

API Key Management

• Secure storage for: Replicate, ElevenLabs, Anthropic (Claude), OpenAI (optional)
• API key validation on entry
• Per-service connection status indicator
• API key rotation support
• Keychain integration (macOS Keychain, iOS Keychain)

Cost Tracking

• Per-generation cost estimate (before generation)
• Actual cost tracking per generation (from API response)
• Per-scene cost summary
• Per-project cost total
• Daily/weekly/monthly spend dashboard
• Cost breakdown by service (Replicate, ElevenLabs, Anthropic)
• Budget alerts: warn when approaching a set limit
• Export cost report as CSV
• Token usage tracking for Claude API calls

Acceptance Criteria

• API keys are stored securely in OS keychain (not plaintext)
• Connection status shows green/red per service
• Cost estimates display before each generation attempt
• Actual costs are recorded and match API billing
• Budget alert triggers notification when threshold reached
• CSV export contains all generation events with costs

Tech Notes

• Use flutter_secure_storage for keychain-backed key storage
• Never log or persist raw API keys in plaintext
• Cost data should be queryable by date range and project

Related

Part of the ai-video-editor Flutter application. Cross-cutting concern used by #65 (Video Generation), #66 (Text-to-Image), #67 (Voice Generation), and #68 (Storytelling Agent).