feat: Add external chat/discussion URL field to groups and events

[tompscanlan]

Summary

Generalize the current Matrix-specific chat integration to a generic "discussion URL" field on groups and events. Organizers can paste a link to any chat platform (Roomy, Matrix, Discord, Telegram, etc.).

What to build

API (this repo)

• Add chatUrl (nullable string, validated URL) to group and event entities
• Add to create/update DTOs and validation
• Migration to add the column

Platform (openmeet-platform)

• Admin/organizer: text input for discussion URL in group/event settings
• Members: "Join Discussion" button that opens the URL in a new tab
• Detect known platforms (roomy.chat, discord.gg, t.me, etc.) to show appropriate icon/label

Design decisions

• Coexists with existing Matrix integration (matrixRoomId) — if chatUrl is set, show it; if not, fall back to embedded Matrix chat
• OpenMeet doesn't manage the room, membership, or provisioning — it's just a link
• For Roomy links, a badge could signal "seamless login" since both apps share ATProto identity (nice-to-have)

Acceptance criteria

• Groups and events have an optional chatUrl field
• Organizers can set/clear the URL from settings
• Members see a "Join Discussion" button when URL is set
• URL is validated (must be a valid URL)
• Existing Matrix chat integration continues to work when chatUrl is not set