MIL-005: Step 4 - Save progress

Review record RC-027 (Go, autonomous mode, not independent).

Task: MIL-005#1
Task: MIL-005#2
Task: MIL-005#3
Task: MIL-005#4
Task: MIL-005#5

src/flash_card/domain/repositories/progress_repository.py

@@ -0,0 +1,29 @@
+"""Port to the words still to learn.
+
+@file progress_repository.py
+@brief The ProgressRepository interface.
+"""
+
+from typing import Protocol
+
+from flash_card.domain.entities.deck import Deck
+
+
+class ProgressRepository(Protocol):
+    """Store of the words still to learn."""
+
+    def load(self) -> Deck | None:
+        """Load the saved words to learn.
+
+        @return The deck, or None when no progress was saved.
+        @throws VocabularyError When the saved progress is unusable.
+        """
+        ...
+
+    def save(self, deck: Deck) -> None:
+        """Save the words still to learn.
+
+        @param deck The cards still to learn.
+        @return Nothing.
+        """
+        ...