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-pgp-spring-boot-starter/pom.xml

@@ -0,0 +1,26 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <parent>
+        <groupId>io.github.sneakybugmaster.mailsender</groupId>
+        <artifactId>mailsender-parent</artifactId>
+        <version>0.1.0-SNAPSHOT</version>
+    </parent>
+
+    <artifactId>mailsender-pgp-spring-boot-starter</artifactId>
+    <description>Starter: OpenPGP-encrypted mail for Spring Boot</description>
+
+    <dependencies>
+        <dependency>
+            <groupId>io.github.sneakybugmaster.mailsender</groupId>
+            <artifactId>mailsender-pgp-spring-boot-autoconfigure</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-mail</artifactId>
+        </dependency>
+    </dependencies>
+</project>