feat(api): slots.da_block_height + BlockResponse.da_block_height (consumes chain v0.2.3's receipt.da_block_height)

[proofmancer]

Indexer-side companion to chain v0.2.3 (ligate-io/ligate-chain#355). Extracts the new receipt.da_block_height field from each slot's first batch and surfaces it as a top-level field on BlockResponse, powering the explorer's "View on Celenium" deep-link.

What

• New migration migrations/20260518000002_slots_da_block_height.sql: ALTER TABLE slots ADD COLUMN da_block_height BIGINT. Nullable.
• Indexer extraction: renamed extract_slot_proposer → extract_slot_first_batch_facts. Same single first-batch fetch now pulls both receipt.da_address (the existing proposer) and receipt.da_block_height (the new field). Returns a tiny SlotFirstBatchFacts struct.
• Slot upsert: db::upsert_slot gains a da_block_height: Option<i64> parameter. Same COALESCE-preserve semantics as proposer — a re-poll that can't reach batches won't blank a known height.
• BlockResponse.da_block_height: Option<u64> — surfaces the column to API consumers. #[serde(skip_serializing_if = "Option::is_none")] so legacy null rows omit the field rather than emit "da_block_height": null.
• Query plumbing: SlotRow + SlotTuple extended with the new column; all 3 SELECT … FROM slots sites updated to include it.

Backward compatibility

• Old slot rows (ingested before this migration) stay da_block_height = NULL until an optional one-shot backfill walks them. The explorer treats null as "no Celenium link to render" — graceful degrade.
• New slot rows ingested after this PR deploys will have the value populated automatically (assuming the chain is on v0.2.3+, which prod is as of 17:53 UTC today).

Verification

• cargo check --workspace clean Rust-side (the local build fails on the risc0-sys Metal kernel script — macOS-only issue, same as every other api PR this week; Linux CI validates).
• Migration applied cleanly to my local dev Postgres.
• Prod chain emits the field: curl https://rpc.ligate.io/v1/ledger/batches/44857 | jq .receipt.da_block_height → 11403025 (verified live during the v0.2.3 swap).

After merge

1. Railway redeploys ligate-api → runs the migration → indexer starts populating the column on new slots.
2. Backfill is optional and skippable; new blocks have the field, old ones stay null.
3. Follow-up ligate-explorer PR renders a "View on Celenium" pill per block when da_block_height is present.