feat(api): Add envelope endpoint and improve API documentation

- Add GET /envelope/{account_id}/{message_id} endpoint to retrieve message envelope (metadata)
- Add get_envelope_by_id method to ENVELOPE_INDEX_MANAGER for querying single envelope
- Move message_id from query parameter to path parameter for clearer API paths:
  - /message-content/{account_id}/{message_id}
  - /download-message/{account_id}/{message_id}
  - /download-attachment/{account_id}/{message_id}
  - /envelope/{account_id}/{message_id}
- Fix API documentation descriptions to be more accurate:
  - search_messages: Now correctly describes search functionality
  - get_thread_messages: Mentions thread_id requirement
  - proxy endpoints: Fixed copy-paste errors from OAuth2 docs
- Update frontend API client to use new path-based URLs

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/modules/indexer/manager.rs

@@ -741,6 +741,47 @@ impl EnvelopeIndexManager {
         })
     }
 
+    pub async fn get_envelope_by_id(
+        &self,
+        account_id: u64,
+        message_id: u64,
+    ) -> BichonResult<Option<Envelope>> {
+        let searcher = self.create_searcher()?;
+        let f = SchemaTools::envelope_fields();
+
+        let query = BooleanQuery::new(vec![
+            (
+                Occur::Must,
+                Box::new(TermQuery::new(
+                    Term::from_field_u64(f.f_account_id, account_id),
+                    IndexRecordOption::Basic,
+                )),
+            ),
+            (
+                Occur::Must,
+                Box::new(TermQuery::new(
+                    Term::from_field_u64(f.f_id, message_id),
+                    IndexRecordOption::Basic,
+                )),
+            ),
+        ]);
+
+        let docs: Vec<(f32, DocAddress)> = searcher
+            .search(&query, &TopDocs::with_limit(1))
+            .map_err(|e| raise_error!(format!("{:#?}", e), ErrorCode::InternalError))?;
+
+        if let Some((_, doc_address)) = docs.first() {
+            let doc: TantivyDocument = searcher
+                .doc_async(*doc_address)
+                .await
+                .map_err(|e| raise_error!(format!("{:#?}", e), ErrorCode::InternalError))?;
+            let envelope = Envelope::from_tantivy_doc(&doc).await?;
+            Ok(Some(envelope))
+        } else {
+            Ok(None)
+        }
+    }
+
     pub async fn top_10_largest_emails(&self) -> BichonResult<Vec<LargestEmail>> {
         self.reader
             .reload()