Fix Dockerfile conflict markers broken on main (#3)

* Fix conflict markers on main and guard CI against them

PR #2 merged before the Dockerfile cleanup landed, leaving <<<<<<<
markers that break docker compose build. Restore clean Docker files
and fail CI if conflict markers appear in the tree.

* Add CI check that fails on merge conflict markers

* Narrow conflict-marker CI check to <<<<<<< and >>>>>>>

---------

Co-authored-by: Cursor Agent <[email protected]>

.env.example

@@ -2,7 +2,6 @@ AUTH_SECRET=change-me-to-a-long-random-string
 # Optional: path to SQLite file (default ./data/rfid.db)
 # RFID_DB_PATH=./data/rfid.db
 
-<<<<<<< HEAD
 # Docker Compose published port (default 3000)
 # RFID_PORT=3000
 
@@ -11,8 +10,6 @@ AUTH_SECRET=change-me-to-a-long-random-string
 # CREATE_USER_PASSWORD=changeme
 # CREATE_USER_NAME=Admin
 
-=======
->>>>>>> origin/main
 # Optional OIDC (Authentik, Keycloak, Authelia, etc.)
 # AUTH_OIDC_ISSUER=https://sso.example.com/application/o/rfid/
 # AUTH_OIDC_CLIENT_ID=