DB: fix unpacking of fulltext filters in for loop

Fixes error:
```
...
File ".../dp3/database/snapshots.py", line 249, in _prepare_latest_query
    for attr, attr_filter in fulltext_filters:
ValueError: too many values to unpack (expected 2)
```
thrown at `GET /entity/{etype}/get` and `GET /entity/{etype}/count`
endpoints when `fulltext_filters` are not empty.

Most likely just a missed replacement from d8ae2b4.

Author: dbnk0

dp3/database/snapshots.py

@@ -246,7 +246,7 @@ def _prepare_latest_query(
         query["latest"] = True
 
         # Process fulltext filters
-        for attr, attr_filter in fulltext_filters:
+        for attr, attr_filter in fulltext_filters.items():
             fulltext_filter = {"$regex": attr_filter, "$options": "i"}
 
             # EID filter