Make weekly Redis key strategy configurable

[ryz310]

Background

This gem generates Redis keys (e.g. weekly_*) based on UNIX time.

Currently, weekly keys are generated using:

strftime('%YW%W')

This causes an issue when a week spans across a year boundary.
For example, the same logical week results in different keys:

• 2025-12-31 → 2025W52
• 2026-01-01 → 2026W00

PR #264 fixes this by generating weekly keys based on the Monday of the week, so the key remains stable across year boundaries.

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Problem

While the fix is more intuitive, it changes existing Redis key names, which is a breaking change for current users.

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Proposal

Introduce a configuration option to let users choose the weekly key strategy:

Redis::Objects::Periodical.configure do |config|
  # :legacy  -> current behavior (strftime('%YW%W'))
  # :monday  -> week-start (Monday) based strategy (PR #264)
  config.weekly_key_strategy = :legacy
end

• Default: :legacy (backward compatible)
• Future major version may switch default to :monday

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Feedback welcome

• Config naming / API design
• Default behavior preference
• Migration concerns

Any feedback is appreciated.