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

mailsender-demo/src/main/resources/application.yml

@@ -0,0 +1,23 @@
+spring:
+  main:
+    web-application-type: none
+  mail:
+    host: ${SMTP_HOST:localhost}
+    port: ${SMTP_PORT:587}
+    username: ${SMTP_USERNAME:}
+    password: ${SMTP_PASSWORD:}
+    properties:
+      mail.smtp.auth: ${SMTP_AUTH:true}
+      mail.smtp.starttls.enable: ${SMTP_STARTTLS:true}
+      mail.smtp.starttls.required: ${SMTP_STARTTLS:true}
+      mail.smtp.connectiontimeout: 10000
+      mail.smtp.timeout: 10000
+
+demo:
+  to: ${DEMO_TO:test@thinhha.pro}
+  from: ${DEMO_FROM:${SMTP_USERNAME:}}
+  count: ${DEMO_COUNT:1}
+
+logging:
+  level:
+    io.github.sneakybugmaster.mailsender: DEBUG