feat: add feature flags mechanism to livedashboard

[filipecabaco]

What kind of change does this PR introduce?

Adds Feature Flags to Realtime

Resolution order:

1. Tenant specific override (stored as a JSONB map on the tenant record)
2. Global flag value
3. false

Flags are cached per node via FeatureFlags.Cache (backed by Cachex). On mutation, global_revalidate/1 multicasts the updated struct to all cluster nodes via GenRpc. Deletes use distributed_invalidate_cache/1. Cache misses fall through to the DB automatically; nil results are intentionally not cached so new flags are visible immediately

# Global flag — no tenant context needed (e.g. a cluster-wide behaviour toggle)
defmodule Realtime.Broadcast do
  import Realtime.FeatureFlags, only: [enabled?: 1]

  def dispatch(message) do
    if enabled?("new_broadcast_engine"), 
      do: NewEngine.dispatch(message), 
      else: LegacyEngine.dispatch(message)
  end
end

# Per-tenant flag — behaviour differs per tenant (e.g. a beta feature rollout)
defmodule Realtime.Presence do
  import Realtime.FeatureFlags, only: [enabled?: 2]

  def track(tenant, payload) do
    if enabled?("enhanced_presence", tenant.external_id), 
      do: EnhancedPresence.track(payload), 
      else: BasicPresence.track(payload)
  end
end

[coveralls]

Coverage is 91.765% — feat/feature-flag into main. No base build found for main.