Rename the app from tracker to fit

The directory, the repo and the hostname are all fit; the module path,
the image, the namespace and the data file were still tracker. Nothing
is deployed yet, so this is free now and would not be later.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Dockerfile

@@ -14,15 +14,15 @@ COPY . .
 RUN CGO_ENABLED=0 GOOS=linux go build \
         -trimpath \
         -ldflags="-s -w" \
-        -o /out/tracker ./cmd/server
+        -o /out/fit ./cmd/server
 
 
 FROM scratch
 
-COPY --from=build /out/tracker /tracker
+COPY --from=build /out/fit /fit
 
 # nobody:nogroup — the only writable path is the mounted volume at /data.
 USER 65534:65534
 
 EXPOSE 8080
-ENTRYPOINT ["/tracker"]
+ENTRYPOINT ["/fit"]