Rename project from TopDeck to Canon

- Module github.com/cdnutter/canon, binary/command cmd/canon, Docker
  image and compose project "canon"
- Environment variables now use the CANON_ prefix
- Default site title is "<Your name>'s Canon" (or "Canon" with no name);
  a Site title set in Settings still overrides it
- Existing topdeck.db (and WAL files) is renamed to canon.db on start
- README: upgrade notes, including moving the old compose volume

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Dockerfile

@@ -6,18 +6,18 @@ COPY go.mod go.sum ./
 RUN go mod download
 COPY . .
 ARG VERSION=dev
-RUN CGO_ENABLED=0 go build -trimpath -ldflags "-s -w -X main.version=${VERSION}" -o /out/topdeck ./cmd/topdeck
+RUN CGO_ENABLED=0 go build -trimpath -ldflags "-s -w -X main.version=${VERSION}" -o /out/canon ./cmd/canon
 
 FROM alpine:3
 RUN apk add --no-cache ca-certificates tzdata \
- && adduser -D -u 1000 topdeck \
- && mkdir -p /data && chown topdeck:topdeck /data
-COPY --from=build /out/topdeck /usr/local/bin/topdeck
-USER topdeck
-ENV TOPDECK_DATA_DIR=/data \
-    TOPDECK_PORT=4242
+ && adduser -D -u 1000 canon \
+ && mkdir -p /data && chown canon:canon /data
+COPY --from=build /out/canon /usr/local/bin/canon
+USER canon
+ENV CANON_DATA_DIR=/data \
+    CANON_PORT=4242
 VOLUME /data
 EXPOSE 4242
 HEALTHCHECK --interval=30s --timeout=3s --start-period=5s \
-  CMD wget -qO- http://127.0.0.1:${TOPDECK_PORT}/healthz >/dev/null || exit 1
-ENTRYPOINT ["topdeck"]
+  CMD wget -qO- http://127.0.0.1:${CANON_PORT}/healthz >/dev/null || exit 1
+ENTRYPOINT ["canon"]