feat: complete AI assistant with custom services and brand config
Baseline snapshot of custom AI implementation before Laravel AI SDK migration.
Includes:
- Custom services: GeminiTextGenerationService, TextGenerationService (OpenAI), ImageGenerationService, AudioGenerationService, VideoGenerationService
- IntentDetector for content moderation via keyword matching
- AI enums: Intent, Orientation, UsageType
- Blade prompt templates: system.blade.php, image.blade.php, video.blade.php
- AiMessage with content_html accessor (markdown rendering)
- AiUsageLog for monthly quota tracking per account
- PostAssistantController with regex-based [GENERATE_*] parsing
- WritingAssistantTab with markdown rendering, add-to-post, attachments
- Workspace brand fields (name, description, tone, voice_notes) in system prompt
- Session state block injected into prompts (thread counts, quota remaining)
- AttachmentCollector pattern will replace the regex approach in Phase 2
- Post comments with replies, emoji reactions, real-time via Echo
- Assets page with Unsplash + Giphy integrations
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