feat: add authentication and book upload

- Session-based login (username/password, 30-day cookie)
- SQLite user store with bcrypt password hashing (modernc.org/sqlite)
- Per-user upload permission (can_upload flag)
- Admin CLI (cmd/admin) for user management:
  user add/list/delete/set-upload
- Upload handler for EPUB/PDF with path-traversal protection
- All routes protected by requireAuth middleware;
  /upload additionally requires requireUpload
- Login/logout UI, upload form, logout button in header
- New env var: USERS_DB (default: users.db, gitignored)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

static/styles.css

@@ -208,3 +208,39 @@ code {
   border: 1px solid #000;
   padding: 0.1rem 0.3rem;
 }
+
+/* --- Auth & Upload --- */
+
+.btn-logout {
+  background: none;
+  border: 1px solid #000;
+  cursor: pointer;
+  padding: 0.3rem 0.6rem;
+  font-size: 0.9rem;
+}
+
+.login-box {
+  max-width: 22rem;
+  margin-top: 3rem;
+}
+
+.login-form,
+.upload-form {
+  display: flex;
+  flex-direction: column;
+  gap: 0.5rem;
+}
+
+.login-form input,
+.upload-form input[type="file"] {
+  padding: 0.5rem;
+  font-size: 1rem;
+  border: 1px solid #000;
+}
+
+.error {
+  border: 2px solid #000;
+  padding: 0.5rem 0.75rem;
+  margin-bottom: 0.75rem;
+  font-weight: bold;
+}