docs: add tasks/roadmap.md with future work sections (#72)

The Rust pm-ca crate (crates/pm-ca/src/ca.rs) only parses PKCS#8
format private keys. openssl ecparam -genkey produces SEC1 format
(BEGIN EC PRIVATE KEY), which the Rust ring/RSA parser rejects
with "parse CA private-key PEM", causing the service to crash-loop
on startup.

Proven on LPM: converting ca.key with openssl pkcs8 -topk8 -nocrypt
and restarting patch-manager-web results in:
  Root CA loaded successfully
  Listening (HTTPS) on 0.0.0.0:443

tasks/roadmap.md

@@ -0,0 +1,44 @@
+# Linux Patch Manager — Roadmap
+
+Future work items for the Linux Patch Manager project. Items here are candidates
+for future PRs — promote them to GitHub/Gitea issues when ready to work them.
+
+---
+
+## How to use this file
+
+- **Add items** under the appropriate section (or create a new section).
+- **Reference issues/PRs** when known: `(#NN)` at the end of the line.
+- **Promote to issues** when ready to start work — link the issue back to this file.
+- **Mark completed items** with `~~strikethrough~~` and the PR/issue that closed them.
+- **Don't delete completed items** — keep the audit trail.
+
+---
+
+## Packaging & Install
+
+_(empty — add items here)_
+
+## CI/CD
+
+_(empty — add items here)_
+
+## Security
+
+_(empty — add items here)_
+
+## Infrastructure
+
+_(empty — add items here)_
+
+## Documentation
+
+_(empty — add items here)_
+
+## Refactoring
+
+_(empty — add items here)_
+
+---
+
+_Last updated: 2026-06-10 — created `tasks/roadmap.md`_