Include newer providers in automatic family sync (#934)

* Include newer providers in automatic family sync

Coinbase, CoinStats, Mercury, and SnapTrade all implement Syncable
and have Syncer classes but were not listed in child_syncables,
meaning their data only refreshed on manual sync button clicks.

* refactor(syncer): Open/Closed principle for provider sync

- Adding new providers requires modifying child_syncables (violates O/C)
- plaid_items missing .active scope (bug: syncs deleted items)
- snaptrade_items can exist without user registration → fails on sync
- Scattered knowledge about 'ready to sync' logic

1. **Registry pattern**: SYNCABLE_ITEM_ASSOCIATIONS constant lists all
   provider associations that participate in family sync

2. **Encapsulated sync-readiness**: Each item model defines its own
   `syncable` scope that knows when it's ready for auto-sync:
   - Most providers: `syncable = active` (not scheduled for deletion)
   - SnapTrade: `syncable = active + user_registered` (has API creds)

3. **Single loop**: child_syncables iterates the registry, calling
   `.syncable` on each association

- Adding a provider = add to registry + define syncable scope
- Each model owns its 'ready to sync' business logic
- Fixes plaid_items bug (now uses .active via .syncable)
- Fixes snaptrade auto-sync failures (filters unregistered items)
- Easy to extend with new conditions per provider

- family/syncer.rb: Registry + dynamic collection
- *_item.rb (7 files): Add `scope :syncable, -> { active }`
- snaptrade_item.rb: Add syncable with user_registered filter

* Fix rubocop bracket spacing in SnaptradeItem syncable scope

app/models/enable_banking_item.rb

@@ -23,6 +23,7 @@ class EnableBankingItem < ApplicationRecord
   has_many :accounts, through: :enable_banking_accounts
 
   scope :active, -> { where(scheduled_for_deletion: false) }
+  scope :syncable, -> { active }
   scope :ordered, -> { order(created_at: :desc) }
   scope :needs_update, -> { where(status: :requires_update) }