Add app version number, shown in Navbar next to the logo

New backend/VERSION file (starts at 1.0.0) served via GET /api/version,
displayed as a small "v1.0.0" next to the StreamHoard wordmark. Going
forward I'll bump this on meaningful changes -- patch for fixes, minor
for new features, major for big/breaking redesigns.

backend/VERSION

@@ -0,0 +1 @@
+1.0.0

backend/server.py

@@ -67,6 +67,13 @@ app = FastAPI(title="StreamHoard")
 api = APIRouter(prefix="/api")
 bearer = HTTPBearer(auto_error=False)
 
+APP_VERSION = (ROOT_DIR / "VERSION").read_text().strip() if (ROOT_DIR / "VERSION").is_file() else "0.0.0"
+
+
+@api.get("/version")
+async def get_version():
+    return {"version": APP_VERSION}
+
 
 # ---------- Auth deps ----------
 async def get_current_user(creds: Optional[HTTPAuthorizationCredentials] = Depends(bearer)) -> dict: