ci(ci): widen Elixir/OTP matrix and run on all branches [PLA-3132]

[davidjulien]

Summary

• Widen the test matrix to Elixir 1.16–1.20 / OTP 25–29 (previously 1.16–1.18 / 25–27), with exclusions for combinations that aren't supported, and ImageOS: ubuntu24 on setup-beam so the newer OTP releases resolve on ubuntu-latest.
• Drop the branches: [main] restriction on push/pull_request so checks run on every branch and PR, matching chargebeex's convention.

Notes

• Kept 1.16.x/OTP 25–26 rather than dropping them to match chargebeex's floor (1.17/OTP 27) — open PR chore(deps): update [mix] ecto_sqlite3 (~> 0.17 → ~> 0.25) #88 (ecto_sqlite3 ~> 0.25 bump) currently passes CI on 1.16, and mix.exs still declares elixir: "~> 1.16", so there's no evidence that support needs to be dropped. Happy to narrow the floor in a follow-up if we want to standardize on chargebeex's exact range.
• The exclusion list for the new OTP 28/29 and Elixir 1.19/1.20 rows is based on Elixir's published compatibility table; this PR's own CI run is the first real verification of those combinations.

Test plan

• CI matrix runs green across all included combinations
• actionlint passes on the workflow file (confirmed locally)

[linear-code]

PLA-3132