docs: Add Rustdoc comments for various structs and functions

Co-authored-by: aider (openai/andrew/openrouter/sonoma-sky-alpha) <aider@aider.chat>

src/services/meta_service.rs

@@ -142,6 +142,21 @@ impl MetaService {
         }
     }
 
+    /// Internal helper to process a meta plugin response and store metadata.
+    ///
+    /// Iterates over the metadata entries in the response and stores each in the database
+    /// using `store_meta`. Logs warnings if storage fails.
+    ///
+    /// # Arguments
+    ///
+    /// * `conn` - Database connection.
+    /// * `item_id` - Item ID to associate with the metadata.
+    /// * `_plugin` - Reference to the plugin (unused).
+    /// * `response` - The plugin response containing metadata.
+    ///
+    /// # Errors
+    ///
+    /// Logs warnings for individual storage failures but does not return errors.
     fn process_plugin_response(
         &self,
         conn: &Connection,
@@ -163,6 +178,21 @@ impl MetaService {
         }
     }
 
+    /// Collects initial metadata from environment variables and hostname.
+    ///
+    /// Gathers metadata from `KEEP_META_*` environment variables and adds hostname
+    /// if not already present.
+    ///
+    /// # Returns
+    ///
+    /// A `HashMap` of initial metadata key-value pairs.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// let service = MetaService::new();
+    /// let initial_meta = service.collect_initial_meta();
+    /// ```
     pub fn collect_initial_meta(&self) -> HashMap<String, String> {
         let mut item_meta: HashMap<String, String> = crate::modes::common::get_meta_from_env();
 
@@ -176,6 +206,11 @@ impl MetaService {
 }
 
 impl Default for MetaService {
+    /// Provides a default `MetaService` instance.
+    ///
+    /// # Returns
+    ///
+    /// A new `MetaService` via `new()`.
     fn default() -> Self {
         Self::new()
     }