Speedy, auditable, deadlock-resistant storage tracking (#2026)

An append-only storage ledger replaces deadlock-prone synchronous
counter updates and drift-prone async updates.

All content storage (card images, card/comment/board description embeds)
is tracked. Account exports (which expire) and avatars are not tracked.

Storage is accounted for by Account and Board. Both consume the same
event stream independently: no bubble-up storage bumps triggering
deadlocks due to lock sequencing. Each calculates its own total from
the underlying ledger with independent cursors and materialization.

* Storage::Entry: Append-only ledger recording attach/detach/transfer
  events with delta bytes. Single event stream indexed for both
  Account and Board cursor queries.

* Storage::Total: Polymorphic snapshot cache with cursor (last_entry_id)
  tracking which entries have been materialized.

* Storage::Totaled concern: Provides bytes_used (fast snapshot) and
  bytes_used_exact (snapshot + pending) query modes, plus
  materialize_storage! to roll up pending entries.

* Storage::Tracked concern: For models owning attachments (Card,
  Comment, Board). Provides board_for_storage_tracking for models
  where board is determined differently (Board returns self).
  Handles board transfers by recording transfer_out/transfer_in entries.

* Storage::AttachmentTracking: Hooks ActiveStorage::Attachment lifecycle
  to record attach/detach entries. Handles ActionText::RichText embeds
  by traversing to the actual model. Snapshots context in before_destroy
  to handle cascading deletes where parent record may be gone by
  after_destroy_commit.

* MaterializeJob: Rolls up pending entries into snapshot. Concurrency
  limited per owner to prevent duplicate work.

* ReconcileJob: On-demand reconciliation against actual attachment
  storage for support/debugging. Compares ledger total to real bytes
  from card images, card embeds, comment embeds, and board embeds.

Usage:
* account.bytes_used / board.bytes_used: fast, slightly stale bytesize
* account.bytes_used_exact / board.bytes_used_exact: real-time bytesize
* Storage::Entry: audit trail for debugging and point-in-time queries

config/initializers/action_text.rb

@@ -11,6 +11,11 @@ module ActionText
           end
         end
       end
+
+      # Delegate storage tracking to the parent record (Card, Comment, Board, etc.)
+      def storage_tracked_record
+        record.try(:storage_tracked_record)
+      end
     end
   end
 end

config/initializers/active_storage.rb

@@ -1,3 +1,7 @@
+ActiveSupport.on_load(:active_storage_attachment) do
+  include Storage::AttachmentTracking
+end
+
 ActiveSupport.on_load(:active_storage_blob) do
   ActiveStorage::DiskController.after_action only: :show do
     expires_in 5.minutes, public: true