refactor(makefile): rename backend → api, fold raw uvicorn target in

The old bare `api` target (uvicorn only) is subsumed into the new `api`
target (db + postgres-wait + migrate + fuser-restart + uvicorn). Updated
all doc references and cleaned up duplicate entries left by the rename.

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

README.md

@@ -88,7 +88,7 @@ Health check: `make check`
 ### Shortcut
 
 ```bash
-make backend  # db + migrate + api (restarts if already running)
+make api  # db + migrate + api (restarts if already running)
 ```
 
 ### Dashboard