feat: implement JSON-driven conversational flow engine

Replaces hardcoded ConversationHandlers with a generic flow engine that reads conversation definitions from talia_bot/data/flows.json.

- Adds a 'conversations' table to the database to persist user state, making flows robust against restarts.
- Implements a central 'universal_handler' in main.py to process all user inputs (text, voice, callbacks, documents) through the new engine.
- Refactors Vikunja, LLM, and Calendar modules to be asynchronous and support the new architecture.
- Adds a new 'transcription' module for OpenAI Whisper and a 'mailer' module for the print flow.
- Implements the full logic for all specified user flows, including project/task management, calendar blocking, idea capture (with branching logic), and the RAG-based client sales funnel.
- Cleans up legacy code and handlers.

requirements.txt

@@ -7,3 +7,4 @@ google-auth-oauthlib
 openai
 pytz
 python-dotenv
+python-dateutil