[ADD] discuss_ai_search: AI-based natural language search for Discuss

[pasaw-odoo]

Discuss's built-in message search only matches literal keywords/substrings in message text. It misses relevant messages that use different wording than the search query, and gives no way to ask a question and get back the matching conversation in context.

This adds an AI search panel to the Discuss sidebar that lets the user type a natural-language prompt and get back the relevant messages from the channel, even when they don't contain the exact words typed:

• discuss_channel.action_ask_ai sends the prompt with the channel's message history to an LLM (Gemini), which returns a natural-language answer plus the ids of the messages that actually match the intent of the prompt, so results stay linked to the real conversation instead of just literal keyword hits.
• discuss_channel.action_summarize_ai adds a one-click AI summary of the channel (short paragraph + key-points bullet list) for quickly catching up on long threads.
• static/src/thread_action.js and static/src/ai_search_panel.js wire both actions into a new Discuss sidebar panel.

This makes Discuss search understand intent rather than exact wording, surfacing relevant chats even when the search prompt doesn't share the same words as the original messages.

[robodoo]