MIL-003: Step 2 - New flash cards

Review record RC-025 (Go, autonomous mode, not independent).

Task: MIL-003#1
Task: MIL-003#2
Task: MIL-003#3
Task: MIL-003#4
Task: MIL-003#5

src/flash_card/domain/entities/card.py

@@ -0,0 +1,17 @@
+"""A flash card.
+
+@file card.py
+@brief The Card entity: a word and its translation.
+"""
+
+from dataclasses import dataclass
+
+
+@dataclass(frozen=True, slots=True)
+class Card:
+    """A word and its translation."""
+
+    ## @brief The word in the front language.
+    front: str
+    ## @brief The translation in the back language.
+    back: str