Add exercise numbers, image uploads, version display, session resume, and training sparklines

- Exercise number (UF#): optional field on exercises, displayed in cards, training, and sets
- Import training plan numbers via migration 005 (UPDATE by name)
- Exercise images: JPG upload with multi-image support per exercise (migration 006)
- Version endpoint (GET /api/v1/version) with ldflags injection in Makefile and Dockerfile
- Version displayed on settings page
- Session resume: GET /api/v1/sessions/active endpoint, auto-resume on training page load
- Block new session while one is active (409 Conflict)
- e1RM sparkline chart per exercise during training (Epley formula)
- Fix CORS: add X-User-ID to allowed headers

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

backend/migrations/006_add_exercise_images.up.sql

@@ -0,0 +1,9 @@
+CREATE TABLE exercise_images (
+    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    exercise_id INTEGER NOT NULL REFERENCES exercises(id),
+    filename TEXT NOT NULL,
+    sort_order INTEGER NOT NULL DEFAULT 0,
+    created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+
+CREATE INDEX idx_exercise_images_exercise_id ON exercise_images(exercise_id);