feat: protect write endpoints with JWT auth (story #08)

Add bcrypt user file auth, JWT middleware on all write routes, Pinia
authStore with localStorage persistence, login view with redirect
support, and v-if guards on all CRUD controls and admin nav link.

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

README.md

@@ -9,15 +9,19 @@ A full-stack fruit-variety database (Go + Vue 3).
 - Administrators can bulk-import the legacy fruit database from XML using `scripts/import_fruits.py`, and then import all publications (cover images, linked fruits, PDFs, fruit images) using `scripts/import_publications.py`.
 - Users can search fruits by name or synonym (case-insensitive, debounced) and filter by type or combined-type alias (e.g. "Birnen- und Quittensorten") from the fruit list.
 - Users can manage publications (books, catalogues) with cover images, linked fruits, per-fruit PDF descriptions, and fruit images; fruit detail pages show publication descriptions and images.
+- Maintainers can log in with a username and password to access data-modifying operations; all write API endpoints and admin controls are protected behind JWT authentication.
 
 ## Quick Start
 
 ```bash
-cp .env.example .env          # set credentials
-make dev-db                   # start Postgres (waits until healthy)
-make migrate-up               # apply schema migrations
-make run-backend              # Echo API on :3000
-make run-frontend             # Vite dev server on :5000
+cp .env.example .env                      # set credentials
+cp backend/.env.example backend/.env      # set JWT_SECRET and USERS_FILE
+cp users.env.example users.env            # create user file (see below)
+./scripts/create_user.sh admin secret >> users.env   # add a user
+make dev-db                               # start Postgres (waits until healthy)
+make migrate-up                           # apply schema migrations
+make run-backend                          # Echo API on :3000
+make run-frontend                         # Vite dev server on :5000
 ```
 
 ## Testing