Add PostService with Business Logic

[cameronhotchkies]

Create business logic layer for posts including markdown rendering to safe HTML.

✅ Acceptance Criteria

• Unit test: creating post with markdown body produces sanitized HTML
• Unit test: markdown containing <script> renders as escaped/safe HTML
• Unit test: update post re-renders markdown to HTML
• Unit test: only post author can update their post (throws on mismatch)
• Unit test: only post author can delete their post

Internal Coding

MP2

• core/posts/PostService.kt — create, update, delete logic with author validation
• core/posts/MarkdownRenderer.kt — CommonMark to sanitized HTML (strips scripts, unsafe tags)