Add BQL query method to FavaClient

Implemented query_bql() method to enable efficient Beancount Query Language
(BQL) queries against Fava API. This is the foundation for replacing manual
balance aggregation (115 lines) with optimized BQL queries.

Benefits:
- Efficient server-side filtering and aggregation
- 5-10x expected performance improvement
- Cleaner, more maintainable code

Next: Implement get_user_balance_bql() using this method.

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

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

fava_client.py

@@ -491,6 +491,61 @@ class FavaClient:
             logger.error(f"Fava connection error: {e}")
             raise
 
+    async def query_bql(self, query_string: str) -> Dict[str, Any]:
+        """
+        Execute arbitrary Beancount Query Language (BQL) query.
+
+        This is a general-purpose method for executing BQL queries against Fava/Beancount.
+        Use this for efficient aggregations, filtering, and data retrieval.
+
+        Args:
+            query_string: BQL query (e.g., "SELECT account, sum(position) WHERE account ~ 'User-abc'")
+
+        Returns:
+            {
+                "rows": [[col1, col2, ...], ...],
+                "types": [{"name": "col1", "type": "str"}, ...],
+                "column_names": ["col1", "col2", ...]
+            }
+
+        Example:
+            result = await fava.query_bql("SELECT account, sum(position) WHERE account ~ 'User-abc'")
+            for row in result["rows"]:
+                account, balance = row
+                print(f"{account}: {balance}")
+
+        See:
+            https://beancount.github.io/docs/beancount_query_language.html
+        """
+        try:
+            async with httpx.AsyncClient(timeout=self.timeout) as client:
+                response = await client.get(
+                    f"{self.base_url}/query",
+                    params={"query_string": query_string}
+                )
+                response.raise_for_status()
+                result = response.json()
+
+                # Fava returns: {"data": {"rows": [...], "types": [...]}}
+                data = result.get("data", {})
+                rows = data.get("rows", [])
+                types = data.get("types", [])
+                column_names = [t.get("name") for t in types]
+
+                return {
+                    "rows": rows,
+                    "types": types,
+                    "column_names": column_names
+                }
+
+        except httpx.HTTPStatusError as e:
+            logger.error(f"BQL query error: {e.response.status_code} - {e.response.text}")
+            logger.error(f"Query was: {query_string}")
+            raise
+        except httpx.RequestError as e:
+            logger.error(f"Fava connection error: {e}")
+            raise
+
     async def get_account_transactions(
         self,
         account_name: str,