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

README.md

@@ -0,0 +1,43 @@
+# mailsender
+
+OpenPGP encryption for Spring Boot mail: discovers recipients' keys via Web Key Directory (WKD) and sends
+PGP/MIME (RFC 3156), or hands mail to an encrypting relay. See [docs/PLAN.md](docs/PLAN.md) for the design
+and milestone status.
+
+Requires JDK 27 and Maven 3.9+. Built on Spring Boot 4.1.1 and PGPainless 2.0.
+
+```
+mvn verify
+```
+
+## Modules
+
+| Module | Contents |
+|---|---|
+| `mailsender-pgp-core` | WKD key discovery, caching, pinned keys (no Spring) |
+| `mailsender-pgp-spring-boot-autoconfigure` | `mail.pgp.*` properties and discovery beans |
+| `mailsender-pgp-spring-boot-starter` | dependency bundle for applications |
+| `mailsender-demo` | sample app that sends test mails |
+
+## Demo app
+
+The demo app looks up the recipient's key via WKD, logs the result, and sends `DEMO_COUNT` test mails
+(default: 1 to `test@thinhha.pro`). Encryption is not wired in yet, so the mails are sent **in plaintext**,
+and the body says whether a key was found.
+
+```
+mvn -q install -DskipTests
+SMTP_HOST=smtp.example.com SMTP_PORT=587 \
+SMTP_USERNAME=me@example.com SMTP_PASSWORD='app-password' \
+DEMO_TO=test@thinhha.pro DEMO_COUNT=1 \
+java -jar mailsender-demo/target/mailsender-demo-0.1.0-SNAPSHOT.jar
+```
+
+| Variable | Default | Meaning |
+|---|---|---|
+| `SMTP_HOST` / `SMTP_PORT` | `localhost` / `587` | SMTP server |
+| `SMTP_USERNAME` / `SMTP_PASSWORD` | empty | SMTP credentials |
+| `SMTP_AUTH` / `SMTP_STARTTLS` | `true` / `true` | set to `false` for a local test server |
+| `DEMO_TO` | `test@thinhha.pro` | recipient |
+| `DEMO_FROM` | `SMTP_USERNAME` | sender address |
+| `DEMO_COUNT` | `1` | number of mails |