feat: add centralized configuration system with Pydantic

- Add backend/config.py with Pydantic settings validation
- Support for standalone, provider, and hybrid operation modes
- Multi-database backend configuration (SQLite/PostgreSQL/MariaDB)
- Environment variable validation with helpful error messages
- Worker and Whisper model configuration

.env.example

@@ -0,0 +1,6 @@
+WHISPER_MODEL=medium
+WEBHOOKPORT=9000
+TRANSCRIBE_DEVICE=gpu
+DEBUG=True
+CLEAR_VRAM_ON_COMPLETE=False
+APPEND=False