sinks: detect silent Wealthfolio /import drops

After the check step returns isValid=true + no errors, a row can still
be silently dropped by /import (response returns activities=[] on
200 OK). Root-cause is usually a field that check hydrates but
/import re-normalises differently (date string form, asset_id resolution).

When we send N valid rows and get back 0, raise ImportValidationError
with a snippet of the check output + first warning — gives the
operator a concrete hint to fix the producer instead of silently
growing dedup against activities that never landed.

poetry run pytest -q   →  109 passed, 1 skipped
poetry run mypy        →  clean
poetry run ruff check  →  clean

broker_sync/sinks/wealthfolio.py

@@ -224,7 +224,24 @@ class WealthfolioSink:
         if isinstance(raw, dict) and "activities" in raw:
             got = raw["activities"]
             assert isinstance(got, list)
-            return got
-        if isinstance(raw, list):
-            return raw
-        return []
+        elif isinstance(raw, list):
+            got = raw
+        else:
+            got = []
+        # Silent-drop detection: if we sent N valid rows but got 0 back, something
+        # is silently rejecting them (usually a date-format or asset-resolution
+        # quirk that check() didn't catch). Raise so the pipeline records failure
+        # instead of marking the rows as synced when they never landed.
+        if valid_rows and not got:
+            # Also surface any per-row `errors` or `warnings` from the check step
+            # — those are often the best hint about why /import dropped them.
+            first_warn = next(
+                (r.get("warnings") for r in checked if isinstance(r, dict) and r.get("warnings")),
+                None,
+            )
+            raise ImportValidationError(
+                f"Wealthfolio /import silently dropped all {len(valid_rows)} rows. "
+                f"First checked row: {checked[0] if checked else 'none'}. "
+                f"First warning (if any): {first_warn}"
+            )
+        return got