Fix .deb install path and Gitea upload auth

The .deb install instructions in the README pointed at a URL that
doesn't exist — Gitea exposes the Debian registry as an apt repo, not
as plain file downloads. Switched the README to the apt-repo flow
(add a sources.list line, then apt install).

Also fixed build-deb.sh: Gitea's Debian package endpoint returns
HTTP 405 for token-bearer auth; it requires HTTP basic auth (user +
token-as-password) and the literal /upload suffix on the URL.

Package built and pushed to the registry — apt install works now.

debian/build-deb.sh

@@ -99,14 +99,14 @@ if [[ "$NO_UPLOAD" == "true" ]]; then
 fi
 
 info "Uploading to Gitea package registry..."
-UPLOAD_URL="$GITEA_URL/api/packages/$GITEA_OWNER/debian/pool/$DISTRO/$COMPONENT"
+# Gitea's Debian registry requires HTTP basic auth (user + token-as-password)
+# and the literal /upload endpoint — token-bearer auth returns 405.
+UPLOAD_URL="$GITEA_URL/api/packages/$GITEA_OWNER/debian/pool/$DISTRO/$COMPONENT/upload"
 
 HTTP_STATUS=$(curl -s -o /tmp/gitea_upload_response.txt -w "%{http_code}" \
-    -X PUT \
-    -H "Authorization: token $GITEA_TOKEN" \
-    -H "Content-Type: application/octet-stream" \
-    --data-binary @"$BUILD_DIR/$DEB_FILE" \
-    "$UPLOAD_URL/$DEB_FILE")
+    -u "$GITEA_OWNER:$GITEA_TOKEN" \
+    --upload-file "$BUILD_DIR/$DEB_FILE" \
+    "$UPLOAD_URL")
 
 if [[ "$HTTP_STATUS" == "201" || "$HTTP_STATUS" == "200" ]]; then
     ok "Uploaded successfully (HTTP $HTTP_STATUS)"