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.

src/error.rs

@@ -40,6 +40,14 @@ pub enum Error {
     /// Failed to parse configuration file.
     #[error("invalid config in {path}: {message}")]
     Config { path: PathBuf, message: String },
+
+    /// Failed to load templates.
+    #[error("failed to load templates: {message}")]
+    TemplateLoad { message: String },
+
+    /// Failed to render template.
+    #[error("failed to render template '{template}': {message}")]
+    TemplateRender { template: String, message: String },
 }
 
 /// Result type alias for compiler operations.