Depend on ecto instead of ecto_sql

[Flo0807]

What

Backpex declares {:ecto_sql, "~> 3.6"}, but nothing from :ecto_sql is used.

Evidence

The library uses Ecto.Query, Ecto.Changeset and Ecto.Repo — all of which live in the :ecto application, not :ecto_sql. :ecto_sql adds the SQL adapters, migrations and mix ecto.* tasks on top.

The only occurrences of an :ecto_sql module are inside documentation:

$ grep -rnE "Ecto\.Adapters|Ecto\.Migration|Ecto\.Migrator" lib/
lib/backpex/fields/upload.ex:205:        use Ecto.Migration
lib/backpex/fields/upload.ex:331:        use Ecto.Migration
lib/backpex/fields/upload.ex:449:        use Ecto.Migration

All three sit inside @moduledoc examples showing users how to write the migration for an upload field — they are documentation text, never compiled. Backpex's own test suite does not use a repo at all, and no adapter is configured in config/.

Impact

This is the only user-facing part: Backpex no longer pulls ecto_sql in transitively, only ecto.

In practice nothing changes — any application using Backpex has an Ecto.Repo backed by a SQL adapter, and therefore already declares ecto_sql itself. The demo does exactly that (demo/mix.exs:73) and compiles unchanged against this branch. What goes away is Backpex constraining the version of a package it never touches, which is what turned number's stale decimal pin into a blocked ecto_sql 3.14 update (#2113).

If you would rather keep the declaration as a convenience for users, the alternative is to leave ecto_sql in place — happy to close this.

Verification

• mix lint passes end to end (compile --warning-as-errors, deps.unlock --unused, format, credo, 100 doctests + 174 tests, 0 failures).
• mix docs --warnings-as-errors builds.
• The demo resolves and compiles cleanly, with ecto_sql still in demo/mix.lock via its own declaration.

Context

Found by a dependency check I am preparing separately, which reports dependencies that are declared but never referenced by the compiled code — the blind spot mix deps.unlock --check-unused cannot cover, since it only compares mix.lock against mix.exs.