license: document grace period + corrected warning intervals

[martinrode]

Summary

Documents the license-management changes that shipped with fylr 6.34.0
(Trac #76801):

• Subscription licenses are now purely time-based (end date + 2-month
  grace period). The previous "binary older than one year" rule no
  longer applies to subscriptions; it is dropped from the docs.
• The 2-month grace period after a subscription's end date is
  documented (definition, grace_to = end date + 2 calendar months,
  shown in the license manager next to the end date).
• The four warning mails are listed: paid-period-ending,
  grace-period-ending, license-expired (past grace), and binary-too-new
  (buy). Dates shown as DD.MM.YYYY.
• The pre-end-date warning intervals in
  for-administrators/readme/license-management.md are corrected from
  30/5/1 days to 30/7/1 days, to match the code
  (expireEmailWarnValidTo in
  internal/license/validation/license_validation.go). The previous
  page had a stale interval.
• The grace-period reminders are added: weekly from 8 weeks before
  grace end, daily in the last week (matches
  expireEmailWarnValidToGrace in the same file).

Notes for review

• Code is authoritative: the intervals, the 2-month grace duration and
  the grace_to formula were taken from internal/license/license.go
  (GraceMonths = 2, GraceTo()) and
  internal/license/validation/license_validation.go (the two
  expireEmailWarn… slices). No discrepancies vs. the spec in the
  6.34.0 release-notes pending-notes block.
• The customer-facing names in the warning-mail bullets
  (paid-period-ending, grace-period-ending, license-expired,
  binary-too-new) mirror the four expire_type / template branches
  the code emits.
• Supersedes the 6.32.0 release-notes line about "accept old binary
  (>1 year) for buy licenses"; that exception is now the only rule
  for buy licenses, and the >1-year-old rule is gone for subscriptions.

Request channel

Sebastian Klarmann assigned Trac #76801 (comment:82) to martin-ai
asking for the docs to be written so they can be folded into the
existing structure. The PR fulfils that request.

Please review:

• @sebastian — owner of the original ticket and the license-lifecycle rework.
• @Charlotte — docs owner.

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