Add members_hash for O(1) direct room lookup

[ashwin47]

Problem

Finding existing DM rooms scans ALL direct rooms in memory (O(n)):

# FIXME: Find a more performant algorithm that won't be a problem                                                        
# on accounts with 10K+ direct rooms                                                                                     
all.joins(:users).detect do |room|                                                                                       
  Set.new(room.user_ids) == Set.new(users.pluck(:id))                                                                    
end                                                                                                                      

Solution

Add indexed members_hash column for O(1) lookup:

• Hash computed from sorted user IDs: MD5("1,3,9")
• Instant room discovery regardless of room count
• Fallback to user matching for gradual backfill

Impact Area

Starting a DM conversation (Rooms::DirectsController#create)

• When clicking a user to start a direct message
• When creating a group DM with multiple users

Migration

• Adds members_hash column with index
• Backfills existing Direct rooms automatically