feat: implement AI source selection and toggle for document-based knowledge (AIIA-66)

Add source scope (case/global) to documents, enabling users to select
which uploaded documents the AI considers during analysis. Includes
schema migration, API support, reusable source selection UI component,
and integration into the analyse form.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

drizzle/0004_document_source_scope.sql

@@ -0,0 +1,17 @@
+-- Add source scope to documents table (AIIA-66)
+-- Documents are either case-specific or globally available
+
+DO $$ BEGIN
+  CREATE TYPE "document_source_scope" AS ENUM ('case', 'global');
+EXCEPTION
+  WHEN duplicate_object THEN null;
+END $$;
+
+ALTER TABLE "documents"
+  ADD COLUMN IF NOT EXISTS "source_scope" "document_source_scope" NOT NULL DEFAULT 'case';
+
+-- Auto-set global scope for norm documents (they are always globally available)
+UPDATE "documents" SET "source_scope" = 'global' WHERE "category" = 'norm';
+
+-- Index for efficient source scope queries
+CREATE INDEX IF NOT EXISTS "documents_source_scope_idx" ON "documents" ("source_scope");