Migrate three pure-function modules from Elixir to Gleam:
- polling_offset: deterministic hash-based polling offset (pure Gleam, no FFI)
- changelog_parser: text parsing logic in Gleam, thin Elixir wrapper for File I/O
- user_agent_parser: regex-based UA parsing in Gleam with Erlang FFI for
atom-keyed map conversion
Add gleam_regexp dependency for regex support in Gleam modules.
Update all call sites and tests to use Gleam module atoms.
Reviewed-on: graham/towerops-web#103
