fix: correct slug generation and add review test coverage

Fix slug generation in forge_webhook version detection to use
repo_url instead of pr_url. The pr_url contains the pull/merge
request number as its last path segment (e.g., /pull/42), which
produced incorrect slugs like '42-42-v2' instead of 'repo-42-v2'.

Add test coverage identified by adversarial review:
- find_previous_version: returns None for v1, cross-thread subject
  matching via author+subject signal
- get_previous_version_findings: returns empty when no prior version
- format_previous_findings: basic formatting, empty input, token
  budget truncation

Signed-off-by: derekbarbosa <derekasobrab@gmail.com>

Author: derekbarbosa

src/api.rs

@@ -1189,7 +1189,7 @@ async fn forge_webhook(
         // Different commit range — new version
         let new_version = existing_version + 1;
 
-        let slug = metadata.pr_url.as_ref().map(|url| {
+        let slug = metadata.repo_url.as_ref().map(|url| {
             let repo = crate::forge::extract_repo_name_from_url(url);
             format!("{}-{}-v{}", repo, metadata.pr_number, new_version)
         });

src/db.rs

@@ -6850,4 +6850,83 @@ mod tests {
         let prev_id: Option<i64> = row.get(0).ok();
         assert_eq!(prev_id, None, "v2 with no matching v1 should have no chain");
     }
+
+    #[tokio::test]
+    async fn test_find_previous_version_returns_none_for_v1() {
+        let db = setup_db().await;
+        let thread_id = db
+            .create_thread("root-fpv", "FPV Test", 1000)
+            .await
+            .unwrap();
+
+        // find_previous_version for v1 should always return None
+        let result = db
+            .find_previous_version(thread_id, "author@test.com", "[PATCH 1/1] Fix", 1)
+            .await
+            .unwrap();
+        assert_eq!(result, None, "v1 should never have a previous version");
+    }
+
+    #[tokio::test]
+    async fn test_find_previous_version_cross_thread_subject_match() {
+        let db = setup_db().await;
+
+        // Create v1 in thread 1
+        let thread1 = db
+            .create_thread("root-ct1", "CT Test 1", 1000)
+            .await
+            .unwrap();
+        db.create_message(
+            "ct-v1",
+            thread1,
+            None,
+            "dev@kernel.org",
+            "[PATCH 1/1] Fix scheduler",
+            1000,
+            "",
+            "",
+            "",
+            None,
+            None,
+        )
+        .await
+        .unwrap();
+        let ps_v1 = db
+            .create_patchset(
+                thread1,
+                None,
+                "ct-v1",
+                "[PATCH 1/1] Fix scheduler",
+                "dev@kernel.org",
+                1000,
+                1,
+                2,
+                "",
+                "",
+                Some(1),
+                1,
+                None,
+                true,
+                None,
+                None,
+            )
+            .await
+            .unwrap()
+            .unwrap();
+
+        // Search for v1 from a different thread by same author + stripped subject
+        let thread2 = db
+            .create_thread("root-ct2", "CT Test 2", 2000)
+            .await
+            .unwrap();
+        let result = db
+            .find_previous_version(thread2, "dev@kernel.org", "[PATCH v2 1/1] Fix scheduler", 2)
+            .await
+            .unwrap();
+        assert_eq!(
+            result,
+            Some(ps_v1),
+            "Should find v1 via author + stripped subject across threads"
+        );
+    }
 }