Bugfix/group invite expiry

[QuickMythril]

Fixes #285 - Bug: GROUP_INVITE expiration not enforced (invites never expire)

Summary

• Add the groupInviteExpiryHeight feature trigger (enum entry + getter) and wire it through mainnet, testnet, and all test-chain fixtures; startup now validates the trigger presence.
• Implement invite expiry enforcement in Group.join(...) gated by the trigger: invite-first joins use join tx timestamps with an inclusive boundary and expiry == null sentinel; expired invites are treated as absent and fall back to join requests; pre-trigger behavior is unchanged. Join-first auto-approvals remain TTL-agnostic (any invite approves a pending request).
• Apply chain-tip-based invite filtering to API endpoints via a shared helper in GroupsResource (inclusive boundary, expiry == null never expires, skip when no tip) and document the behavior in swagger.
• Update docs (INVITE_EXPIRATION.md, IMPLEMENTATION.md, CONSENSUS_CHANGE.md) with final semantics, activation plan, join-first clarifications, and activation rollout guidance.
• Add comprehensive tests: invite-first expiry enforcement, join-first TTL-agnostic behavior, backdated/forward-dated windows, pre/post-trigger activation, and API invite filtering (chain-tip filtering and no-tip fallback). Includes a dedicated GroupInviteFilteringApiTests class.

Tests

• mvn -q -DskipTests=false -Dtest=org.qortal.test.group.MiscTests test
• mvn -q -DskipTests=false -Dtest=org.qortal.test.api.GroupInviteFilteringApiTests test

Follow-up for activation

• Choose and set a real groupInviteExpiryHeight for mainnet once network coverage is sufficient; keep the placeholder until then.
• Communicate the activation height/date to operators and include it in release notes/changelog.
• Verify testnet/fixture triggers remain at immediate activation for ongoing coverage, adjusting only if the activation scheme changes.

[nbenaglia]

Docs of this PR should be gathered in a dedicated folder ("group_invite_expiry" for example).
If the docs folder grows up with more documents, it's very difficult to understand to what feature the single document refers to.