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/routes/dashboard.py

@@ -1,19 +1,26 @@
 """Progress dashboard routes.
 
-Displays summary statistics, volume charts, and per-exercise progress.
+Displays summary statistics, volume charts, per-exercise progress,
+and CSV export of workout history.
 """
 
+import csv
+import io
 import json
+import re
+from datetime import date, timedelta
+from typing import Optional
 
 import structlog
-from fastapi import APIRouter, Depends, Request
-from fastapi.responses import HTMLResponse
+from fastapi import APIRouter, Depends, Query, Request
+from fastapi.responses import HTMLResponse, StreamingResponse
 from sqlmodel import Session
 
 from app.database import get_db_session
 from app.models.user import User
 from app.services.analytics_service import AnalyticsService
 from app.services.exercise_service import ExerciseService
+from app.services.export_service import ExportService
 from app.services.progression_service import ProgressionService
 from app.utils.auth import require_active_profile
 
@@ -36,6 +43,10 @@ async def dashboard(
     exercise_service = ExerciseService(session)
     exercises = exercise_service.list_exercises()
 
+    today = date.today()
+    export_start = (today - timedelta(days=30)).isoformat()
+    export_end = today.isoformat()
+
     templates = request.app.state.templates
     return templates.TemplateResponse("pages/dashboard.html", {
         "request": request,
@@ -43,9 +54,52 @@ async def dashboard(
         "volume_data_json": json.dumps(volume_data),
         "exercises": exercises,
         "active_profile": profile,
+        "export_start_date": export_start,
+        "export_end_date": export_end,
     })
 
 
+@router.get("/export")
+async def export_csv(
+    request: Request,
+    start_date: Optional[str] = Query(None),
+    end_date: Optional[str] = Query(None),
+    session: Session = Depends(get_db_session),
+    profile: User = Depends(require_active_profile),
+):
+    """Export workout history as a CSV file download."""
+    today = date.today()
+    default_start = today - timedelta(days=30)
+
+    try:
+        parsed_start = date.fromisoformat(start_date) if start_date else default_start
+    except ValueError:
+        parsed_start = default_start
+
+    try:
+        parsed_end = date.fromisoformat(end_date) if end_date else today
+    except ValueError:
+        parsed_end = today
+
+    export_service = ExportService(session)
+    rows = export_service.get_export_rows(profile.id, parsed_start, parsed_end)
+
+    output = io.StringIO()
+    headers = ["date", "workout_type", "exercise", "set_number", "reps", "weight", "felt_easy"]
+    writer = csv.DictWriter(output, fieldnames=headers)
+    writer.writeheader()
+    writer.writerows(rows)
+
+    safe_name = re.sub(r"[^a-z0-9_]", "", profile.display_name.lower().replace(" ", "_"))
+    filename = f"sneakyswole_{safe_name}_{parsed_start.isoformat()}_to_{parsed_end.isoformat()}.csv"
+
+    return StreamingResponse(
+        iter([output.getvalue()]),
+        media_type="text/csv",
+        headers={"Content-Disposition": f'attachment; filename="{filename}"'},
+    )
+
+
 @router.get("/exercise/{exercise_id}", response_class=HTMLResponse)
 async def exercise_progress(
     exercise_id: int,