feat: add Stalwart Mail Server and FoundationDB library packages

- add stalwart: main mail server with systemd integration and user management
- add stalwart-cli: command line interface tool for server administration
- add stalwart-foundationdb: FoundationDB-enabled version with proper dependencies
- add libfoundationdb: FoundationDB client library (libfdb_c.so) package

All packages include proper conflict resolution, systemd services, and follow
repository packaging conventions. stalwart and stalwart-foundationdb are
mutually exclusive to prevent installation conflicts.

rpms/stalwart-foundationdb/resources/nfpm.yaml

@@ -0,0 +1,50 @@
+# nfpm.yaml
+
+name: stalwart-foundationdb
+version: ${PACKAGE_VERSION}
+release: ${PACKAGE_RELEASE}
+arch: amd64
+platform: linux
+section: default
+priority: extra
+description: "Stalwart Mail Server with FoundationDB support - A modern, secure JMAP, IMAP and SMTP mail server"
+
+maintainer: Stalwart Labs
+homepage: https://stalw.art
+license: AGPL-3.0
+
+disable_globbing: false
+
+replaces:
+  - stalwart-foundationdb
+
+provides:
+  - stalwart-foundationdb
+
+conflicts:
+  - stalwart
+
+depends:
+  - libfoundationdb
+
+# Files to include in the package
+contents:
+  - src: /app/stalwart-foundationdb
+    dst: /opt/stalwart/bin/stalwart-foundationdb
+    file_info:
+      mode: 0755
+      owner: stalwart
+      group: stalwart
+  - src: /app/resources/stalwart-foundationdb.service
+    dst: /usr/lib/systemd/system/stalwart-foundationdb.service
+    file_info:
+      mode: 0644
+      owner: root
+      group: root
+
+# Scripts to run during installation/removal
+scripts:
+  preinstall: /app/resources/scripts/preinstall.sh
+  postinstall: /app/resources/scripts/postinstall.sh
+  preremove: /app/resources/scripts/preremove.sh
+  postremove: /app/resources/scripts/postremove.sh