Expand the completeness checker with more number field, Dirichlet, and local field cases (LMFDB#7017) - #31

Add several new provable-completeness cases to lmfdb/utils/completeness.py,
each with a message and a test using the example query from issue LMFDB#7017:

- Number fields: a degree-2 field unramified outside S has |disc| dividing
  8*prod_{odd p in S} p (factor 8 only if 2 in S), so it is complete when that
  product is within the degree-2 discriminant bound (new NFBound.clear_S_disc).
- Number fields: a CM field is totally imaginary, constraining the signature to
  [0, n/2] (n even), which e.g. reduces degree 2 + CM to the class-number bound.
- Number fields: for imaginary quadratic fields r_1 = 0, so the narrow class
  number/group equals the ordinary one.
- Dirichlet characters: a primitive character has conductor equal to its modulus,
  so a conductor bound bounds the modulus.
- p-adic fields: discriminant exponent c = 0 means unramified (e = 1), giving
  n = f via MulFiller (new UnramifiedFiller).

All additions are gated on specific query keys and use string reasons, leaving
clear_S/display_reason/to_rset untouched. Verified with the flask test client on
the issue's example queries (before: no message; after: completeness flash) and
via lmfdb/tests/test_utils.py::test_complete (new complete and not-complete cases),
which passes against devmirror; pyflakes clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

…ess-cases