Refactor default headers to be injected dynamically (#367)

crates/yaak-models/migrations/20260119045146_remove-default-workspace-headers.sql

@@ -0,0 +1,12 @@
+-- Filter out headers that match the hardcoded defaults (User-Agent: yaak, Accept: */*),
+-- keeping any other custom headers the user may have added.
+UPDATE workspaces
+SET headers = (
+    SELECT json_group_array(json(value))
+    FROM json_each(headers)
+    WHERE NOT (
+        (LOWER(json_extract(value, '$.name')) = 'user-agent' AND json_extract(value, '$.value') = 'yaak')
+        OR (LOWER(json_extract(value, '$.name')) = 'accept' AND json_extract(value, '$.value') = '*/*')
+    )
+)
+WHERE json_array_length(headers) > 0;