fava_client: resolve relative target_file paths against the ledger root

Fava's /api/source endpoint rejects relative paths with HTTP 500
(NonSourceFileError: "Trying to read a non-source file at '...'"). The
include-aware `_infer_target_file` helper returns relative paths
(e.g. "accounts/users.beancount"), so add a `_resolve_target_file`
hook that prepends the ledger root directory.

The dirname is derived from a one-time GET /api/options and cached on
the FavaClient instance (which is a module-level singleton), guarded by
an asyncio.Lock so concurrent first-callers don't double-fetch.

Absolute paths pass through unchanged, so the admin endpoint that
explicitly passes target_file="accounts/chart.beancount" works the same
as one that passes "/var/lib/fava/accounts/chart.beancount".

Verified against aio-demo's live fava: relative paths now produce
HTTP 200 reads on options.beancount, accounts/chart.beancount,
accounts/users.beancount, and transactions.beancount.

Refs: aiolabs/libra#28

fava_client.py

@@ -80,6 +80,46 @@ class FavaClient:
         # Per-user locks for user-specific operations (reduces contention)
         self._user_locks: Dict[str, asyncio.Lock] = {}
 
+        # Cached absolute dirname of the root ledger file, derived from
+        # GET /api/options on first need. Used by `_resolve_target_file` to
+        # turn relative include paths (e.g. "accounts/users.beancount") into
+        # the absolute paths fava's /api/source endpoint requires.
+        self._main_dir_cache: Optional[str] = None
+        self._main_dir_lock = asyncio.Lock()
+
+    async def _resolve_target_file(self, target_file: str) -> str:
+        """
+        Turn a relative include path into the absolute path fava expects.
+
+        Fava's /api/source endpoint refuses relative paths with HTTP 500
+        (NonSourceFileError). Resolve any non-absolute target_file by
+        prepending the directory of the root ledger file (cached after
+        the first GET /api/options).
+
+        Args:
+            target_file: Relative (e.g. "accounts/users.beancount") or
+                absolute path.
+
+        Returns:
+            Absolute path under fava's ledger root.
+        """
+        import os
+
+        if os.path.isabs(target_file):
+            return target_file
+
+        if self._main_dir_cache is None:
+            async with self._main_dir_lock:
+                if self._main_dir_cache is None:
+                    async with httpx.AsyncClient(timeout=self.timeout) as client:
+                        resp = await client.get(f"{self.base_url}/options")
+                        resp.raise_for_status()
+                        main_file = resp.json()["data"]["beancount_options"]["filename"]
+                    self._main_dir_cache = os.path.dirname(main_file)
+                    logger.debug(f"Cached fava ledger root dir: {self._main_dir_cache}")
+
+        return os.path.join(self._main_dir_cache, target_file)
+
     def get_user_lock(self, user_id: str) -> asyncio.Lock:
         """
         Get or create a lock for a specific user.
@@ -1560,6 +1600,9 @@ class FavaClient:
         if target_file is None:
             target_file = _infer_target_file(account_name)
 
+        # Fava's /api/source requires absolute paths; convert if needed.
+        target_file = await self._resolve_target_file(target_file)
+
         last_error = None
 
         for attempt in range(max_retries):