feat: answer what is like this section or instructor

[jaspermayone]

Part of #453. Stacked on #640.

Two endpoints that answer "what is like this one?":

curl "…/api/v1/catalog/sections/17294/similar?limit=5"
curl "…/api/v1/catalog/instructors/fac_kw7coe30/similar"

and the same on GraphQL:

{ section(crn: 17294) { similar(limit: 5) { crn title } } }

Two choices worth reviewing:

• Similar sections stay in the section's own term. A student picks from
  what is offered now, and the same course in another term carries identical
  text, so it would always win.
• The other sections of the same course are left out. COMP 1050-02 is the
  nearest thing to COMP 1050-01 and the least useful answer to the question.

limit is 10 by default and 50 at most, in both surfaces. The GraphQL fields
carry complexity: 10 and a @listSize directive, because each one runs its
own vector query; the published cost stays honest.

The list is empty until the record has a vector, which the nightly backfill
writes.

Tests

bundle exec rspec is green locally (1552 examples), including the empty
cases, the limits, and the 404s.

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