[Omi] Auto-discard conversations under 30 seconds and remove toggle

## Changes **Backend:** - Hard-discard conversations under 30 seconds of total duration OR under 30 seconds of actual speaking time - Do this as a simple duration check before the LLM discard prompt — no need to waste an LLM call for these - In `process_conversation.py`, add an early return before `should_discard_conversation()` if duration < 30s **App:** - Remove the "Show short conversations" toggle from conversation display settings - Remove `showShortConversations` preference and `shortConversationThreshold` - Remove the client-side duration filter in `conversation_provider.dart` - These are no longer needed since short convos will be discarded server-side instead of hidden client-side ## Why - Short convos (<30s) are noise — filler words, brief greetings, accidental recordings - Currently relying on LLM to decide discard, which is inconsistent and wastes API calls - The client-side "show short conversations" toggle has a threshold of 0, making it useless - Cleaner to discard at