feat: add Phase 3 Workout UI — auth, profiles, workout viewer, exercise browser

Build the core user-facing experience with admin login (bcrypt + signed
session cookies), profile switcher, workout day viewer with warmups and
exercise cards, and HTMX-powered exercise browser with search/filter.

- AuthService with bcrypt password verification and itsdangerous session tokens
- Auth dependency redirects to /login (303) for unauthenticated requests
- NavContextMiddleware injects admin/profiles/active_profile into all templates
- Profile management (list, switch, edit) with cookie-based active profile
- Workout day viewer shows warmups + exercises + per-user programming targets
- Exercise browser with HTMX filter dropdowns (no page reloads)
- Flash message partial for success/error feedback
- 12 new tests (66 total passing)

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

app/routes/exercises.py

@@ -0,0 +1,86 @@
+"""Exercise browser routes with HTMX search/filter support.
+
+All filtering is done via HTMX partial responses — no JSON APIs.
+"""
+
+import structlog
+from fastapi import APIRouter, Depends, Query, Request
+from fastapi.responses import HTMLResponse
+from sqlmodel import Session
+
+from app.database import get_db_session
+from app.models.user import User
+from app.services.exercise_service import ExerciseService
+from app.utils.auth import get_current_admin_user
+
+logger = structlog.get_logger(__name__)
+
+router = APIRouter(prefix="/exercises", tags=["exercises"])
+
+
+@router.get("", response_class=HTMLResponse)
+async def exercise_browser(
+    request: Request,
+    session: Session = Depends(get_db_session),
+    admin: User = Depends(get_current_admin_user),
+):
+    """Render the exercise browser page with all exercises.
+
+    Args:
+        request: The incoming HTTP request.
+        session: Database session.
+        admin: The authenticated admin user.
+
+    Returns:
+        Rendered exercise browser page.
+    """
+    exercise_service = ExerciseService(session)
+    exercises = exercise_service.list_exercises()
+    workout_days = exercise_service.list_workout_days()
+
+    # Collect unique muscle groups for the filter dropdown
+    muscle_groups = sorted(set(ex.muscle_group for ex in exercises))
+
+    templates = request.app.state.templates
+    return templates.TemplateResponse("pages/exercise_browser.html", {
+        "request": request,
+        "exercises": exercises,
+        "workout_days": workout_days,
+        "muscle_groups": muscle_groups,
+        "admin": admin,
+    })
+
+
+@router.get("/search", response_class=HTMLResponse)
+async def exercise_search(
+    request: Request,
+    session: Session = Depends(get_db_session),
+    admin: User = Depends(get_current_admin_user),
+    workout_day: str = Query(default="", alias="workout_day"),
+    muscle_group: str = Query(default="", alias="muscle_group"),
+):
+    """Return filtered exercise list as an HTMX partial.
+
+    Called via hx-get from the exercise browser filter dropdowns.
+
+    Args:
+        request: The incoming HTTP request.
+        session: Database session.
+        admin: The authenticated admin user.
+        workout_day: Filter by workout day name.
+        muscle_group: Filter by muscle group.
+
+    Returns:
+        Rendered exercise list partial HTML.
+    """
+    exercise_service = ExerciseService(session)
+    exercises = exercise_service.list_exercises(
+        workout_day=workout_day or None,
+        muscle_group=muscle_group or None,
+    )
+
+    templates = request.app.state.templates
+    return templates.TemplateResponse("partials/exercise_list.html", {
+        "request": request,
+        "exercises": exercises,
+    })