fix(db): align ON CONFLICT clause with existing functional index

The migration script `V6.8.0.1.sql` failed with a `PSQLException` stating "there is no unique or exclusion constraint matching the ON CONFLICT specification".

This occurred because the script used a partial index syntax for the conflict target:
`ON CONFLICT (name) WHERE lang IS NULL`

However, PostgreSQL's `ON CONFLICT` inference requires the target to syntactically match an existing unique index or constraint. The actual index on the `setting` table is a functional index defined as:
`CREATE UNIQUE INDEX unique_settings ON setting (name, (COALESCE(lang, '')))`

While `WHERE lang IS NULL` and `COALESCE(lang, '')` logically handle nulls similarly for uniqueness in this context, Postgres does not treat them as interchangeable for arbiter inference.

This commit updates the migration script to explicitly target the functional index expression:
`ON CONFLICT (name, (coalesce(lang, '')))`

References:
- PostgreSQL 16 Documentation on INSERT: https://www.postgresql.org/docs/16/sql-insert.html#SQL-ON-CONFLICT

Author: poikilotherm

src/main/resources/db/migration/V6.8.0.1.sql

@@ -67,7 +67,7 @@ DO $$
         -- Insert or update the new JSON-based setting
         INSERT INTO setting (name, content, lang)
         VALUES (':TabularIngestSizeLimit', json_object::TEXT, NULL)
-        ON CONFLICT (name) WHERE lang IS NULL
+        ON CONFLICT (name, (coalesce(lang, '')))
             DO UPDATE SET content = EXCLUDED.content;
 
         -- Delete all format-specific settings