feat(templates): add Tera runtime template engine

Lay groundwork for user-editable templates by adding Tera as a
runtime template engine alongside the existing maud templates.

Changes:
- Add tera dependency
- Create TemplateEngine struct with render methods
- Add TemplateLoad/TemplateRender error variants
- Add section_type/template fields to Frontmatter
- Create templates/ directory with base, page, section, and content templates

Dead code warnings are expected; TemplateEngine will be wired
in to replace maud in subsequent commits.

Cargo.toml

@@ -34,6 +34,7 @@ lightningcss = "1.0.0-alpha.70"
 # Config parsing
 katex-rs = "0.2.3"
 serde    = { version = "1", features = ["derive"] }
+tera     = "1"
 toml     = "0.8"
 
 # Diagram rendering