perf: Eliminate double serialization, add SQLite tuning, optimize hot paths

11 isomorphic performance fixes from deep audit (no behavior changes):

- Eliminate double serialization: store_payload now accepts pre-serialized
  bytes (&[u8]) instead of re-serializing from serde_json::Value. Uses
  Cow<[u8]> for zero-copy when compression is disabled.
- Add SQLite cache_size (64MB) and mmap_size (256MB) pragmas
- Replace SELECT-then-INSERT label upserts with INSERT...ON CONFLICT
  RETURNING in both issues.rs and merge_requests.rs
- Replace INSERT + SELECT milestone upsert with RETURNING
- Use prepare_cached for 5 hot-path queries in extractor.rs
- Optimize compute_list_hash: index-sort + incremental SHA-256 instead
  of clone+sort+join+hash
- Pre-allocate embedding float-to-bytes buffer with Vec::with_capacity
- Replace RandomState::new() in rand_jitter with atomic counter XOR nanos
- Remove redundant per-note payload storage (discussion payload contains
  all notes already)
- Change transform_issue to accept &GitLabIssue (avoids full struct clone)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/documents/extractor.rs

@@ -77,11 +77,18 @@ pub fn compute_content_hash(content: &str) -> String {
 
 /// Compute SHA-256 hash over a sorted list of strings.
 /// Used for labels_hash and paths_hash to detect changes efficiently.
+/// Sorts by index reference to avoid cloning, hashes incrementally to avoid join allocation.
 pub fn compute_list_hash(items: &[String]) -> String {
-    let mut sorted = items.to_vec();
-    sorted.sort();
-    let joined = sorted.join("\n");
-    compute_content_hash(&joined)
+    let mut indices: Vec<usize> = (0..items.len()).collect();
+    indices.sort_by(|a, b| items[*a].cmp(&items[*b]));
+    let mut hasher = Sha256::new();
+    for (i, &idx) in indices.iter().enumerate() {
+        if i > 0 {
+            hasher.update(b"\n");
+        }
+        hasher.update(items[idx].as_bytes());
+    }
+    format!("{:x}", hasher.finalize())
 }
 
 /// Extract a searchable document from an issue.
@@ -132,7 +139,7 @@ pub fn extract_issue_document(conn: &Connection, issue_id: i64) -> Result<Option
     };
 
     // Query labels via junction table
-    let mut label_stmt = conn.prepare(
+    let mut label_stmt = conn.prepare_cached(
         "SELECT l.name FROM issue_labels il
          JOIN labels l ON l.id = il.label_id
          WHERE il.issue_id = ?1
@@ -245,7 +252,7 @@ pub fn extract_mr_document(conn: &Connection, mr_id: i64) -> Result<Option<Docum
     };
 
     // Query labels via junction table
-    let mut label_stmt = conn.prepare(
+    let mut label_stmt = conn.prepare_cached(
         "SELECT l.name FROM mr_labels ml
          JOIN labels l ON l.id = ml.label_id
          WHERE ml.merge_request_id = ?1
@@ -373,7 +380,7 @@ pub fn extract_discussion_document(
                     Err(e) => return Err(e.into()),
                 };
                 // Query parent labels
-                let mut label_stmt = conn.prepare(
+                let mut label_stmt = conn.prepare_cached(
                     "SELECT l.name FROM issue_labels il
                      JOIN labels l ON l.id = il.label_id
                      WHERE il.issue_id = ?1
@@ -407,7 +414,7 @@ pub fn extract_discussion_document(
                     Err(e) => return Err(e.into()),
                 };
                 // Query parent labels
-                let mut label_stmt = conn.prepare(
+                let mut label_stmt = conn.prepare_cached(
                     "SELECT l.name FROM mr_labels ml
                      JOIN labels l ON l.id = ml.label_id
                      WHERE ml.merge_request_id = ?1
@@ -423,7 +430,7 @@ pub fn extract_discussion_document(
         };
 
     // Query non-system notes in thread order
-    let mut note_stmt = conn.prepare(
+    let mut note_stmt = conn.prepare_cached(
         "SELECT n.author_username, n.body, n.created_at, n.gitlab_id,
                 n.note_type, n.position_old_path, n.position_new_path
          FROM notes n
@@ -657,6 +664,7 @@ mod tests {
                 updated_at INTEGER NOT NULL,
                 last_seen_at INTEGER NOT NULL,
                 discussions_synced_for_updated_at INTEGER,
+                resource_events_synced_for_updated_at INTEGER,
                 web_url TEXT,
                 raw_payload_id INTEGER
             );
@@ -899,6 +907,7 @@ mod tests {
                 discussions_sync_last_attempt_at INTEGER,
                 discussions_sync_attempts INTEGER DEFAULT 0,
                 discussions_sync_last_error TEXT,
+                resource_events_synced_for_updated_at INTEGER,
                 web_url TEXT,
                 raw_payload_id INTEGER
             );