feat: add funding event tracking with cursor-based pagination

[pcfreak30]

• Capture actual funded deltas (AccountDeposit + ContractID) from funding loop
• Record funding events to funding_events table with amount_sc, estimated upload/download bytes
• Add cursor-based (created_at, id) pagination for funding history API
• Add GET /funding/events admin endpoint with Go client support
• Add SQL migration for funding_events table with composite indexes
• Calculate estimated bytes from SC amount using host sector costs at funding time
• Cap estimatedBytes overflow at math.MaxInt64

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Add funding event tracking with cursor-based pagination

This PR introduces tracking of account funding events, recording each deposit made during the funding process along with estimated upload and download byte coverage. A new admin API endpoint (GET /funding/events) allows querying these events using cursor-based pagination.

Key changes:

• FundingEvent model: New struct capturing account key, host key, contract ID, amount, estimated upload/download bytes, and creation timestamp
• FundedDeposit type: The FundAccounts method now returns deposit details alongside funded/drained counts, enabling downstream recording of individual deposits
• Event recording: After funding accounts (both service and regular), deposits are converted to funding events with estimated byte coverage (calculated from host pricing assuming a 50/50 read/write split) and persisted
• Cursor-based pagination: New FundingCursor (timestamp + ID) enables stable pagination through events ordered by creation time
• Database: New funding_events table with indexes for account+host lookups, host+time queries, and cursor-based pagination; includes migration
• Admin API: New GET /funding/events endpoint with after, id, and limit query parameters

[kody-ai]

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