Scaffold PGP mail modules with WKD key discovery and demo app

- Maven multi-module build on JDK 27 and Spring Boot 4.1.1 (BOM import)
- core: WKD discovery (advanced -> direct fallback), HTTP fetcher with
  timeouts, no redirects and a size limit, Caffeine cache with per-outcome
  TTLs capped at key expiry, pinned keys, composite chain
- autoconfigure: mail.pgp.* properties and discovery beans
- demo: sends test mails (plaintext for now) and reports the WKD result
- docs/PLAN.md: design, decisions and milestones

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01G71TQ5jDETVwdUJxGAwaiT

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