feat: add CSV export of workout history to dashboard

Add date range picker and download button to the dashboard page,
backed by GET /dashboard/export endpoint that returns a StreamingResponse
CSV file. Completes Phase 4 (final V2 improvement).

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

app/services/export_service.py

@@ -0,0 +1,80 @@
+"""Export service for generating CSV-ready workout history data.
+
+Queries workout logs joined with sessions, days, and exercises
+to produce flat rows suitable for CSV export.
+"""
+
+from datetime import date
+
+import structlog
+from sqlmodel import Session, select
+
+from app.models.exercise import Exercise
+from app.models.workout_log import WorkoutLog
+from app.models.workout_session import WorkoutSession
+
+logger = structlog.get_logger(__name__)
+
+
+class ExportService:
+    """Builds export-ready workout history rows.
+
+    Args:
+        session: An active SQLModel Session.
+    """
+
+    def __init__(self, session: Session) -> None:
+        self._session = session
+
+    def get_export_rows(
+        self, user_id: int, start_date: date, end_date: date,
+    ) -> list[dict]:
+        """Get flat workout log rows for CSV export.
+
+        Args:
+            user_id: The user whose data to export.
+            start_date: Inclusive start of date range.
+            end_date: Inclusive end of date range.
+
+        Returns:
+            List of dicts with keys: date, workout_type, exercise,
+            set_number, reps, weight, felt_easy.
+        """
+        sessions = self._session.exec(
+            select(WorkoutSession)
+            .where(
+                WorkoutSession.user_id == user_id,
+                WorkoutSession.date >= start_date,
+                WorkoutSession.date <= end_date,
+            )
+            .order_by(WorkoutSession.date.asc())
+        ).all()
+
+        if not sessions:
+            return []
+
+        # Pre-load exercises for name lookup
+        exercises = self._session.exec(select(Exercise)).all()
+        exercise_map = {e.id: e for e in exercises}
+
+        rows = []
+        for ws in sessions:
+            logs = self._session.exec(
+                select(WorkoutLog)
+                .where(WorkoutLog.session_id == ws.id)
+                .order_by(WorkoutLog.exercise_id, WorkoutLog.set_number)
+            ).all()
+
+            for log_entry in logs:
+                exercise = exercise_map.get(log_entry.exercise_id)
+                rows.append({
+                    "date": ws.date.isoformat(),
+                    "workout_type": exercise.workout_day if exercise else "Unknown",
+                    "exercise": exercise.name if exercise else "Unknown",
+                    "set_number": log_entry.set_number,
+                    "reps": log_entry.reps_completed,
+                    "weight": log_entry.weight_used,
+                    "felt_easy": "Yes" if log_entry.felt_easy else "No",
+                })
+
+        return rows