fix: resolve Pydantic date field shadowing causing 422 on event update

The field name 'date' in EventUpdate shadowed the 'date' type from
datetime, causing Pydantic v2 to interpret Optional[date] as
Optional[None] — rejecting all non-null values with a 422 error.

Fix: rename import to 'date as _date' and use '_date' in all
annotations. Also improve error handling in admin forms to properly
display 422 validation error messages.

Co-Authored-By: Claude <noreply@anthropic.com>

backend/app/schemas.py

@@ -1,11 +1,22 @@
-from datetime import date
+from datetime import date as _date
 from typing import Optional
 
-from pydantic import BaseModel, field_validator
+from pydantic import BaseModel, computed_field, field_validator
 
 
 # ── Program ──────────────────────────────────────────────
 
+DAY_NAMES = {
+    1: "Monday",
+    2: "Tuesday",
+    3: "Wednesday",
+    4: "Thursday",
+    5: "Friday",
+    6: "Saturday",
+    7: "Sunday",
+}
+
+
 class ProgramCreate(BaseModel):
     day_of_week: int
     day_label: str
@@ -27,19 +38,23 @@ class ProgramUpdate(BaseModel):
 class ProgramResponse(BaseModel):
     id: int
     day_of_week: int
-    day_label: str
     time: str
     title: str
     host: str
     genre: str
 
+    @computed_field
+    @property
+    def day_label(self) -> str:
+        return DAY_NAMES.get(self.day_of_week, "Unknown")
+
     model_config = {"from_attributes": True}
 
 
 # ── Event ────────────────────────────────────────────────
 
 class EventCreate(BaseModel):
-    date: date
+    date: _date
     title: str
     description: str
     location: str
@@ -48,7 +63,7 @@ class EventCreate(BaseModel):
 
 
 class EventUpdate(BaseModel):
-    date: Optional[date] = None
+    date: Optional[_date] = None
     title: Optional[str] = None
     description: Optional[str] = None
     location: Optional[str] = None
@@ -59,7 +74,7 @@ class EventUpdate(BaseModel):
 
 class EventResponse(BaseModel):
     id: int
-    date: date
+    date: _date
     title: str
     description: str
     location: str