refactor(db): rename searchspaces table to workspaces

Phase 1 (rename DB) commit 1i: rename the SearchSpace table to workspaces
and flip all 24 inbound FK target strings (20 in db.py + the 4 satellite
models), plus the raw-SQL searchspaces references in the obsidian and
google-unification integration fixtures.

This completes the ORM half of Phase 1. The SearchSpace class name,
relationship/back_populates attribute names, and the /searchspaces API
route URLs are intentionally left for Phase 2.

Verified: unit 2375 passed/1 skip, integration 346 passed (baseline
parity); create_all builds every table with the workspaces name and all
FKs resolving to workspaces.id.

surfsense_backend/app/file_storage/persistence/models.py

@@ -32,7 +32,7 @@ class DocumentFile(BaseModel, TimestampMixin):
     search_space_id = Column(
         "workspace_id",
         Integer,
-        ForeignKey("searchspaces.id", ondelete="CASCADE"),
+        ForeignKey("workspaces.id", ondelete="CASCADE"),
         nullable=False,
         index=True,
     )