Add codespell support with configuration and fixes

[yarikoptic]

Add codespell-based
spell-checking to Postal and fix existing typos.

I personally introduced codespell to over a hundred projects already,
mostly with positive feedback (see
"improveit-dashboard").
The added GitHub Actions workflow has permissions: contents: read so it
is safe to enable.

What's in this PR

The PR is broken into small, reviewable commits so individual pieces can
be dropped if not desired:

1. Add GitHub Actions workflow — runs codespell on push and
   pull_request against main.

2. Add rudimentary .codespellrc — minimal initial config.

3. Tune .codespellrc — skip auto-generated CHANGELOG.md, DKIM /
   postfix-bounce MIME test fixtures, and the test SSH signing key, all
   of which contain MIME-encoded or binary-ish content that trips the
   spell-check. Add checkin to ignore-words-list since it's a method
   name in lib/postal/message_db/connection_pool.rb, not a misspelling.

4. Fix historical typos in CHANGELOG.md (safe to drop) —
   nexting → nesting (rubocop block-nesting changelog entry) and
   messsages → messages. CHANGELOG.md is now in codespell's skip
   list, so CI passes regardless of whether this commit is kept; this is
   a one-off cleanup of typos that originated in past commit messages
   and were copied verbatim by release-please.

5. Fix ambiguous typos requiring context review — actuall → actually in a spec comment and whent he → when the in an RSpec
   context label. Both required human judgment.

6. Fix non-ambiguous typos found by codespell — auto-corrected by
   codespell -w, 22 files / 43 hits. All in comments, log strings,
   Prometheus docstrings, HAML view prose, RSpec context labels, doc
   pages, and YAML config templates / schema descriptions. No
   behavioural code is altered.

   Most-frequent corrections: overriden → overridden (13x),
   sesssion → session (9x), Somethign → Something (4x),
   mesage/mesages/messsage → message(s) (4x), plus single-instance
   apprpriate, particularily, encouters, successfuly,
   Orginal, whcih, likelyhood, thsi, maxmium, maxmimum,
   faciliate, occurrs, theres.

Notes for reviewers

• The fix in app/lib/message_dequeuer/incoming_message_processor.rb:85
  (maxmimum → maximum in a log string) is mirrored by an update to the
  matching RSpec regex in
  spec/lib/message_dequeuer/incoming_message_processor_spec.rb:133,
  so the regression test still matches.
• In app/lib/smtp_server/server.rb:311, codespell rewrote a Prometheus
  docstring from "successfuly TLS connections established" to
  "successfully TLS connections established". Grammatically the
  author likely meant successful (adjective) — happy to swap if you
  prefer.
• This repo has had 14 prior commits with typo/spell/spelling
  in their subject line, demonstrating that automated spell-checking
  would have saved manual review work.

Testing

codespell passes with zero errors against the tracked tree after all
fixes (uvx codespell → exit 0).

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🤖 Generated with Claude Code and love
for typo-free code.