feat: disallow online scaling for shared source

[xxchan]

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[xxchan]

• feat: disallow online scaling for shared source #18355 👈
• feat: track progress for SourceBackfill (blocking DDL) #18112 : 1 other dependent PR (#16825 )
• refactor: use high watermark to finish backfill faster #18342 : 1 other dependent PR (#18368 )
• refactor: add target_offsets to determinine if source backfill finished #18297
• refactor: simplify SourceBackfill's memory state #18296
• main

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