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

db/migrate/20251205205826_create_storage_tables.rb

@@ -0,0 +1,27 @@
+class CreateStorageTables < ActiveRecord::Migration[8.0]
+  def change
+    # Storage ledger: debit/credit event stream
+    create_table :storage_entries, id: :uuid do |t|
+      t.references :account, type: :uuid, null: false
+      t.references :board, type: :uuid, null: true
+
+      t.references :recordable, type: :uuid, polymorphic: true, null: true
+
+      t.bigint :delta, null: false
+      t.string :operation, null: false
+
+      t.datetime :created_at, null: false
+    end
+
+    # Storage totals: cached snapshots
+    create_table :storage_totals, id: :uuid do |t|
+      t.references :owner, type: :uuid, polymorphic: true, null: false, index: false
+
+      t.bigint :bytes_stored, null: false, default: 0
+      t.uuid :last_entry_id  # Cursor: includes all entries <= this ID
+
+      t.timestamps
+      t.index %i[ owner_type owner_id ], unique: true
+    end
+  end
+end

db/migrate/20251210054934_add_blob_id_and_audit_context_to_storage_entries.rb

@@ -0,0 +1,9 @@
+class AddBlobIdAndAuditContextToStorageEntries < ActiveRecord::Migration[8.2]
+  def change
+    change_table :storage_entries do |t|
+      t.references :blob, type: :uuid, foreign_key: false, index: true
+      t.references :user, type: :uuid, foreign_key: false, index: true
+      t.string :request_id, index: true
+    end
+  end
+end