feat(query): Add stream backlog admin API

[zhyass]

fix

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Summary

Adds a stream backlog admin API for estimating pending stream changes from metadata.

API

• GET /v1/stream_backlog?stream_name=&database=
• GET /v1/tenants/:tenant/stream_backlog?stream_name=&database=

stream_name is required. database is optional and defaults to default.

Result
The API returns:

{
    "rows_added": 3,
    "rows_removed": 0,
    "estimated_rows": 3,
    "estimated_bytes": 120
}

• rows_added: metadata-derived rows from latest-side changes.
• rows_removed: metadata-derived rows from base-side changes. Append-only streams report 0.
• estimated_rows: estimated rows the stream may need to process.
• estimated_bytes: estimated uncompressed bytes for estimated_rows.

The values are metadata-derived estimates, not exact stream output row counts.

Changes

• Computes backlog from Fuse snapshot and block metadata.
• Handles append-only streams, standard update/delete/truncate cases, and compact/recluster-style physical rewrites.
• Extends management tests to cover API output and result fields across append, update, delete, truncate, and compaction behavior.

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is 

[zhyass]

@codex review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. You're on a roll.

Reviewed commit: 11644d5619

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