Refactor database handling to exclusively support PostgreSQL

- Removed SQLite support from the configuration and database initialization logic.
- Updated database migration scripts to focus solely on PostgreSQL migrations.
- Simplified logging messages to reflect PostgreSQL usage.
- Adjusted database schema definitions to remove SQLite-specific types and structures.
- Modified tests to ensure compatibility with PostgreSQL, including changes to table creation and data types.
- Cleaned up unused imports and code related to SQLite.

.env.example

@@ -52,11 +52,7 @@ AI_LLM_MODEL=free
 # NVIDIA Nemotron Content Safety Configuration
 NVIDIA_NEMOTRON_API_KEY=your_nvidia_api_key_here
 
-# Database Configuration
-DATABASE_TYPE=sqlite
-# DATABASE_TYPE=postgres
-
-# PostgreSQL Configuration (used when DATABASE_TYPE=postgres)
+# Database Configuration (PostgreSQL)
 # Option 1: Use DATABASE_URL for connection string
 # DATABASE_URL=postgresql://user:password@localhost:5432/discord_bot