Add vendors to processing activities

Signed-off-by: Sacha Al Himdani <sacha@getprobo.com>

pkg/coredata/migrations/20251030T120000Z.sql

@@ -0,0 +1,8 @@
+CREATE TABLE processing_activity_vendors (
+    processing_activity_id TEXT REFERENCES processing_activities(id) ON DELETE CASCADE,
+    vendor_id TEXT REFERENCES vendors(id) ON DELETE CASCADE,
+    tenant_id TEXT NOT NULL,
+    snapshot_id TEXT,
+    created_at TIMESTAMP WITH TIME ZONE NOT NULL,
+    PRIMARY KEY (processing_activity_id, vendor_id)
+);