feat: define all 8 SQLModel tables

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/models/workout_day.py

@@ -0,0 +1,26 @@
+"""WorkoutDay model for the 4-day training split.
+
+Defines the named workout days and their order.
+"""
+
+from typing import Optional
+
+from sqlmodel import Field, SQLModel
+
+
+class WorkoutDay(SQLModel, table=True):
+    """A named workout day in the training program.
+
+    Attributes:
+        id: Primary key, auto-incremented.
+        name: Day name (Push, Pull, Lower, Full Body).
+        day_number: Order in the weekly rotation (1-4).
+        description: Brief description of the day's focus.
+    """
+
+    __tablename__ = "workout_days"
+
+    id: Optional[int] = Field(default=None, primary_key=True)
+    name: str = Field(index=True, unique=True)
+    day_number: int = Field(unique=True)
+    description: str = Field(default="")