Implement comprehensive LLM provider system with global cost protection

- Add multi-provider LLM architecture supporting OpenRouter, OpenAI, Gemini, and custom providers
- Implement global LLM on/off switch with default DISABLED state for cost protection
- Add per-character LLM configuration with provider-specific models and settings
- Create performance-optimized caching system for LLM enabled status checks
- Add API key validation before enabling LLM providers to prevent broken configurations
- Implement audit logging for all LLM enable/disable actions for cost accountability
- Create comprehensive admin UI with prominent cost warnings and confirmation dialogs
- Add visual indicators in character list for custom AI model configurations
- Build character-specific LLM client system with global fallback mechanism
- Add database schema support for per-character LLM settings
- Implement graceful fallback responses when LLM is globally disabled
- Create provider testing and validation system for reliable connections

migrations/006_add_character_llm_settings.sql

@@ -0,0 +1,18 @@
+-- Add LLM configuration columns to characters table
+-- Migration: 006_add_character_llm_settings.sql
+
+ALTER TABLE characters 
+ADD COLUMN llm_provider VARCHAR(50),
+ADD COLUMN llm_model VARCHAR(100),
+ADD COLUMN llm_temperature FLOAT,
+ADD COLUMN llm_max_tokens INTEGER;
+
+-- Add indexes for common queries
+CREATE INDEX IF NOT EXISTS ix_characters_llm_provider ON characters(llm_provider);
+CREATE INDEX IF NOT EXISTS ix_characters_llm_model ON characters(llm_model);
+
+-- Add comments for documentation
+COMMENT ON COLUMN characters.llm_provider IS 'Per-character LLM provider override (openrouter, openai, gemini, custom)';
+COMMENT ON COLUMN characters.llm_model IS 'Specific model name for this character';
+COMMENT ON COLUMN characters.llm_temperature IS 'Creativity/randomness setting (0.1-2.0)';
+COMMENT ON COLUMN characters.llm_max_tokens IS 'Maximum response length for this character';