fix: attribute 985k blank regions from coordinates, unblocking migration 23

[keonik]

With the guard corrected, migration 23 reported the real obstacle:

985,634 row(s) have no region while rows also exist for 0, BE, IH, IN, MI, O, ON, PA, PJ

About 17% of the table has no state at all.

Why not just stamp them OH

That is the same mistake as the backfill already reverted on this branch, just
quieter. Border-county extracts genuinely contain Indiana, Pennsylvania and
Michigan addresses, and a mislabel is permanent and invisible.

us_states already holds TIGER boundaries behind a GIST index — it is what
/states/lookup answers from — so each blank row is asked where it actually
is and takes the state it sits in.

Verified against a fixture spanning three states:

before:  <blank>:4  OH:1  PA:1  oh:1
after:   OH:4  IN:1  MI:1  PA:1

Migration 23: attributed 4 of 4 blank region(s) from their coordinates

The blanks in Ohio became OH, the one in Indiana became IN, the one in
Michigan became MI. A single stamp could not have been right for all three.

Rows that cannot be attributed

If a row is still blank afterwards, its coordinates fall outside every state
boundary — the location is wrong, not the state missing. The migration names
the count and stops rather than inventing a state:

1 row(s) still have no region after attribution: their coordinates fall outside
every US state boundary, so the location is wrong rather than the state missing.
Fix or delete them, then re-run

Verified with a row in the middle of the Atlantic.

Cost

One pass of ~1M point-in-polygon lookups against a GIST-indexed boundary table,
inside the migration's existing transaction. It holds ROW EXCLUSIVE, so reads
are unaffected; the ACCESS EXCLUSIVE statements still come last.

Related

/coverage reports blank regions as Ohio — it does
COALESCE(NULLIF(region,''),'OH'). That is why nobody saw a million stateless
rows, and why the real Ohio count is nearer 4.79M than the 5.78M it shows. Next
PR fixes that and adds the data-quality endpoint that would have surfaced all
of this directly.

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