feat(import): remember the user's import selection (#619)

Co-authored-by: Claude Opus 5 <noreply@anthropic.com>

crates/yaak-models/migrations/20260902000000_import-source-content-hash.sql

@@ -0,0 +1,24 @@
+-- Replace the per-resource snapshot with a content hash, and let a row exist without a model
+-- so a resource the user chose not to import can be remembered.
+CREATE TABLE import_source_resources_new
+(
+    model            TEXT     DEFAULT 'import_source_resource' NOT NULL,
+    created_at       DATETIME DEFAULT CURRENT_TIMESTAMP        NOT NULL,
+    updated_at       DATETIME DEFAULT CURRENT_TIMESTAMP        NOT NULL,
+    import_source_id TEXT                                      NOT NULL,
+    source_key       TEXT                                      NOT NULL,
+    model_type       TEXT                                      NOT NULL,
+    model_id         TEXT,
+    content_hash     TEXT,
+    PRIMARY KEY (import_source_id, source_key)
+);
+
+INSERT INTO import_source_resources_new (model, created_at, updated_at, import_source_id,
+                                         source_key, model_type, model_id, content_hash)
+SELECT model, created_at, updated_at, import_source_id, source_key, model_type, model_id, NULL
+FROM import_source_resources;
+
+DROP TABLE import_source_resources;
+
+ALTER TABLE import_source_resources_new
+    RENAME TO import_source_resources;