Fix ActionCable auth fallback + unblock deploy (diff-lcs)

[HamptonMakes]

Summary

• Unify authentication across HTTP and ActionCable via CoPlan::Authentication
• Fallback to configured authenticate callback when connection.current_user is absent
• Fix deploy failure by adding missing diff-lcs dependency

Why

• Hosts without ApplicationCable::Connection#current_user were failing websocket auth
• diff_to_operations requires diff/lcs but dependency was missing, breaking deploy

Changes

• Add engine/app/services/coplan/authentication.rb
• Update PlanPresenceChannel to resolve user via shared auth
• Update ApplicationController to use shared auth
• Add diff-lcs to gemspec
• Add spec for websocket auth fallback

Verification

• Specs: 83 examples, 0 failures
• Manual: channel fallback resolves user via authenticate callback

Risk

• Low: consolidates existing auth logic; preserves behavior when current_user exists

Follow-ups

• Bump coplan-engine in coplan-square after merge and redeploy