feat: add site_search service with Brave path and domain filtering

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/src/services/mod.rs

@@ -11,6 +11,7 @@ pub mod prompts;
 pub mod rate_limiter;
 pub mod scheduler;
 pub mod scraper;
+pub mod site_search;
 pub mod source_scraper;
 pub mod synthesis;
 pub mod turnstile;

backend/src/services/site_search.rs

@@ -0,0 +1,134 @@
+//! Site-scoped search fallback service.
+//!
+//! When a personalized source yields 0 links from RSS + HTML extraction,
+//! this service searches `site:{domain} {theme}` via Brave Search API
+//! or LLM websearch to discover articles from that source.
+
+use std::sync::Arc;
+
+use crate::services::llm::LlmProvider;
+
+/// Configuration for a site-scoped search.
+pub struct SiteSearchConfig {
+    pub domain: String,
+    pub theme: String,
+    pub max_results: usize,
+    pub max_age_days: i32,
+}
+
+/// Provider for executing the site-scoped search.
+pub enum SiteSearchProvider {
+    /// Use the Brave Search API.
+    Brave { api_key: String },
+    /// Use an LLM with websearch capabilities.
+    Llm {
+        provider: Arc<dyn LlmProvider>,
+        model: String,
+    },
+}
+
+/// Execute a site-scoped search, returning article URLs.
+///
+/// Searches `site:{domain} {theme}` via the configured provider.
+/// Returns an empty Vec on failure (silent fallback — this is a last-resort strategy).
+pub async fn search(
+    http_client: &reqwest::Client,
+    config: &SiteSearchConfig,
+    provider: &SiteSearchProvider,
+) -> Vec<String> {
+    match provider {
+        SiteSearchProvider::Brave { api_key } => {
+            search_brave(http_client, config, api_key).await
+        }
+        SiteSearchProvider::Llm {
+            provider: llm,
+            model,
+        } => search_llm(config, llm, model).await,
+    }
+}
+
+/// Brave Search path: query `site:{domain} {theme}` via the Brave API.
+async fn search_brave(
+    http_client: &reqwest::Client,
+    config: &SiteSearchConfig,
+    api_key: &str,
+) -> Vec<String> {
+    let query = format!("site:{} {}", config.domain, config.theme);
+
+    let results = match crate::services::brave_search::search(
+        http_client,
+        api_key,
+        &query,
+        config.max_results as u32,
+        config.max_age_days,
+    )
+    .await
+    {
+        Ok(results) => results,
+        Err(e) => {
+            tracing::warn!(
+                domain = %config.domain,
+                error = %e,
+                "Site search fallback (Brave) failed"
+            );
+            return Vec::new();
+        }
+    };
+
+    let urls: Vec<String> = results
+        .into_iter()
+        .filter(|r| url_matches_domain(&r.url, &config.domain))
+        .map(|r| r.url)
+        .collect();
+
+    tracing::info!(
+        domain = %config.domain,
+        results = urls.len(),
+        "Site search fallback (Brave) completed"
+    );
+
+    urls
+}
+
+/// Check if a URL belongs to the expected domain.
+fn url_matches_domain(url: &str, expected_domain: &str) -> bool {
+    url::Url::parse(url)
+        .ok()
+        .and_then(|u| u.host_str().map(|h| h.to_lowercase()))
+        .map(|host| host == expected_domain || host.ends_with(&format!(".{}", expected_domain)))
+        .unwrap_or(false)
+}
+
+// Placeholder for LLM path (Task 2)
+async fn search_llm(
+    _config: &SiteSearchConfig,
+    _provider: &Arc<dyn LlmProvider>,
+    _model: &str,
+) -> Vec<String> {
+    Vec::new()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn url_matches_domain_exact() {
+        assert!(url_matches_domain("https://korben.info/article", "korben.info"));
+    }
+
+    #[test]
+    fn url_matches_domain_subdomain() {
+        assert!(url_matches_domain("https://www.korben.info/article", "korben.info"));
+    }
+
+    #[test]
+    fn url_matches_domain_mismatch() {
+        assert!(!url_matches_domain("https://evil.com/korben.info", "korben.info"));
+    }
+
+    #[test]
+    fn url_matches_domain_invalid_url() {
+        assert!(!url_matches_domain("not a url", "korben.info"));
+    }
+}