Backend tests run on SQLite by default (78 passed, 1 skipped); no Postgres needed

- conftest: default SHONAR_TEST_DATABASE_URL is a temp SQLite file, matching
  the bundled-lite engine; set the env var to a PG URL to exercise that path
- models.py: add sqlite_where to the two partial unique indexes — without it
  SQLite built a FULL unique index on recording_id (WHERE not carried over),
  wrongly blocking a second export asset per recording
- test_m9: pass UUID objects (not str) to direct ORM inserts/gets; SQLite's
  GUID bind processor rejects strings (asyncpg tolerated them)

Verified: pytest -q = 78 passed, 1 skipped; ruff check clean

backend/shonar/db/models.py

@@ -26,6 +26,7 @@ from sqlalchemy import (
     String,
     Text,
     UniqueConstraint,
+    text,
 )
 from sqlalchemy.dialects.postgresql import JSONB
 from sqlalchemy.orm import Mapped, mapped_column, relationship
@@ -170,6 +171,7 @@ class Recording(Base, PublicIdMixin):
             "client_recording_id",
             unique=True,
             postgresql_where="client_recording_id IS NOT NULL",
+            sqlite_where=text("client_recording_id IS NOT NULL"),
         ),
     )
 
@@ -204,11 +206,15 @@ class Asset(Base, PublicIdMixin):
 
     __table_args__ = (
         # At most one immutable "original" per recording.
+        # sqlite_where is required as well as postgresql_where: without it
+        # SQLite gets a FULL unique index on recording_id (the WHERE clause
+        # is not carried over), which wrongly blocks >1 export asset/recording.
         Index(
             "uq_assets_one_original",
             "recording_id",
             unique=True,
             postgresql_where="kind = 'original'",
+            sqlite_where=text("kind = 'original'"),
         ),
     )