feat(admin): add potential fathers retrieval for character management

- Implemented a new method in AdminService to fetch potential fathers for a given character based on existing relationships.
- Updated AdminController to expose this functionality via a new API endpoint.
- Enhanced adminRouter to include the route for retrieving potential fathers.
- Modified frontend components to allow selection of potential fathers during pregnancy and birth management.
- Updated internationalization files to include new translation keys related to father selection.

frontend/src/i18n/locales/en/admin.json

@@ -231,17 +231,33 @@
                     "statusActive": "Expecting until",
                     "statusNone": "Not pregnant",
                     "fatherId": "Father character ID (optional)",
+                    "fatherSelect": "Father (spouse / fiancé(e) / lover)",
+                    "fatherNone": "— no father stored —",
+                    "fatherHintList": "From this character’s relationships (marriage, engagement, active affair).",
+                    "fatherHintManual": "No matching partner in the database: enter the father’s character ID manually.",
+                    "fatherManualPlaceholder": "Character ID",
                     "dueDays": "Days until due date",
+                    "dueDaysHint": "0 = due today (birth may follow depending on game logic).",
                     "force": "Set pregnancy",
                     "clear": "Clear pregnancy",
                     "successForce": "Pregnancy has been set.",
                     "successClear": "Pregnancy has been cleared.",
-                    "error": "Action failed."
+                    "error": "Action failed.",
+                    "relationship": {
+                        "married": "Spouse",
+                        "engaged": "Engaged partner",
+                        "lover": "Lover"
+                    }
                 },
                 "birth": {
                     "title": "Force birth (admin)",
                     "motherHint": "The character listed above is used as the mother.",
                     "fatherId": "Father character ID",
+                    "fatherSelect": "Father (spouse / fiancé(e) / lover)",
+                    "fatherChoose": "— choose father —",
+                    "fatherHintList": "From this character’s relationships.",
+                    "fatherHintManual": "No partner in the list: enter the father’s character ID manually.",
+                    "fatherRequired": "Please select a father or enter the character ID.",
                     "context": "Context",
                     "contextMarriage": "Marriage",
                     "contextLover": "Affair",