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/core/payloads.rs

@@ -15,19 +15,18 @@ pub struct StorePayloadOptions<'a> {
     pub project_id: Option<i64>,
     pub resource_type: &'a str, // 'project' | 'issue' | 'mr' | 'note' | 'discussion'
     pub gitlab_id: &'a str,     // TEXT because discussion IDs are strings
-    pub payload: &'a serde_json::Value,
+    pub json_bytes: &'a [u8],
     pub compress: bool,
 }
 
 /// Store a raw API payload with optional compression and deduplication.
 /// Returns the row ID (either new or existing if duplicate).
 pub fn store_payload(conn: &Connection, options: StorePayloadOptions) -> Result<i64> {
-    // 1. JSON stringify the payload
-    let json_bytes = serde_json::to_vec(options.payload)?;
+    let json_bytes = options.json_bytes;
 
     // 2. SHA-256 hash the JSON bytes (pre-compression)
     let mut hasher = Sha256::new();
-    hasher.update(&json_bytes);
+    hasher.update(json_bytes);
     let payload_hash = format!("{:x}", hasher.finalize());
 
     // 3. Check for duplicate by (project_id, resource_type, gitlab_id, payload_hash)
@@ -51,12 +50,12 @@ pub fn store_payload(conn: &Connection, options: StorePayloadOptions) -> Result<
     }
 
     // 5. Compress if requested
-    let (encoding, payload_bytes) = if options.compress {
+    let (encoding, payload_bytes): (&str, std::borrow::Cow<'_, [u8]>) = if options.compress {
         let mut encoder = GzEncoder::new(Vec::new(), Compression::default());
-        encoder.write_all(&json_bytes)?;
-        ("gzip", encoder.finish()?)
+        encoder.write_all(json_bytes)?;
+        ("gzip", std::borrow::Cow::Owned(encoder.finish()?))
     } else {
-        ("identity", json_bytes)
+        ("identity", std::borrow::Cow::Borrowed(json_bytes))
     };
 
     // 6. INSERT with content_encoding
@@ -71,7 +70,7 @@ pub fn store_payload(conn: &Connection, options: StorePayloadOptions) -> Result<
             now_ms(),
             encoding,
             &payload_hash,
-            &payload_bytes,
+            payload_bytes.as_ref(),
         ),
     )?;
 
@@ -143,6 +142,7 @@ mod tests {
     fn test_store_and_read_payload() {
         let conn = setup_test_db();
         let payload = serde_json::json!({"title": "Test Issue", "id": 123});
+        let json_bytes = serde_json::to_vec(&payload).unwrap();
 
         let id = store_payload(
             &conn,
@@ -150,7 +150,7 @@ mod tests {
                 project_id: Some(1),
                 resource_type: "issue",
                 gitlab_id: "123",
-                payload: &payload,
+                json_bytes: &json_bytes,
                 compress: false,
             },
         )
@@ -164,6 +164,7 @@ mod tests {
     fn test_compression_roundtrip() {
         let conn = setup_test_db();
         let payload = serde_json::json!({"data": "x".repeat(1000)});
+        let json_bytes = serde_json::to_vec(&payload).unwrap();
 
         let id = store_payload(
             &conn,
@@ -171,7 +172,7 @@ mod tests {
                 project_id: Some(1),
                 resource_type: "issue",
                 gitlab_id: "456",
-                payload: &payload,
+                json_bytes: &json_bytes,
                 compress: true,
             },
         )
@@ -185,6 +186,7 @@ mod tests {
     fn test_deduplication() {
         let conn = setup_test_db();
         let payload = serde_json::json!({"id": 789});
+        let json_bytes = serde_json::to_vec(&payload).unwrap();
 
         let id1 = store_payload(
             &conn,
@@ -192,7 +194,7 @@ mod tests {
                 project_id: Some(1),
                 resource_type: "issue",
                 gitlab_id: "789",
-                payload: &payload,
+                json_bytes: &json_bytes,
                 compress: false,
             },
         )
@@ -204,7 +206,7 @@ mod tests {
                 project_id: Some(1),
                 resource_type: "issue",
                 gitlab_id: "789",
-                payload: &payload,
+                json_bytes: &json_bytes,
                 compress: false,
             },
         )