[storage/ordinal] fix panic in get after prune fails mid-removal

[0xAysh]

Fixes #2069

Problem

Ordinal::get assumes that if self.intervals contains an index, self.blobs must have the corresponding section:

  let blob = self.blobs.get(&section).unwrap();

prune removes a section in three steps:

  1. self.blobs.remove(&section)    -- remove from in-memory map
  2. context.remove(...).await?     -- delete from disk
  3. self.intervals.remove(...)     -- remove from intervals

If step 2 fails, ? returns early and step 3 never runs. Now blobs is missing the section but intervals still has its indices. The next call to get for one of those indices passes the intervals check and panics on the unwrap.

Fix

Move self.intervals.remove to before context.remove. If the disk deletion fails, both maps are already consistent, and get returns Ok(None) instead of panicking.