[codex] Add signed cursor pagination and query budgets

[prissca]

Summary

• add signed, expiring, versioned cursors bound to authenticated filter scope
• migrate transaction and notification feeds to stable timestamp/id cursor pagination
• add bounded projections, page sizes, Mongo query deadlines, and compound indexes
• document explain-plan verification and query budget operations

Verification

• git diff --check
• TypeScript compilation is blocked because dependencies, including TypeScript, are not installed in this clone

Closes #112

[Obiajulu-gif]

@prissca this looks great.. love it