fix(github): preserve full review comment IDs

[morluto]

Description

GitHub marks PullRequestReviewComment.databaseId as deprecated in favor of the BigInt-backed fullDatabaseId. The feedback synchronizer queried the deprecated field for review-thread comments and converted the returned ID to int64, which would both fail when GitHub removes the field and lose identifiers outside the signed 64-bit range.

This change queries fullDatabaseId in both review-thread paths and carries comment and reply identifiers as strings through the feedback adapter and projection. Existing raw observations containing numeric IDs remain readable.

Testing

• make verify passes
• Focused regression coverage preserves string IDs larger than int64 and checks both GraphQL queries avoid deprecated comment fields
• make test-race passes (not needed; this change does not alter storage concurrency, locking, or cancellation)

Compatibility and scope

• Storage and side-effect invariants remain intact
• No unrelated cleanup or generated-output churn is included

No public API shape changes: persisted feedback IDs were already normalized as strings.