feat: Move media assets outside of the git repository

Images and media referenced by the private Copper Chronicle section are no
longer tracked by git. They live at /assets/copper-chronicle/ on the server
(/srv/www/binning.net/assets/) and are managed independently. Updated all
image links in src/private/copper-chronicle to use absolute paths rooted at
/assets/. Added --filter='protect assets/' to all deploy rsync commands to
prevent accidental deletion during deploys.

.forgejo/workflows/cd.yml

@@ -22,5 +22,5 @@ jobs:
           path: book/
       - name: Deploy
         run: |
-          nix shell nixpkgs#rsync --command rsync -av --delete book/ /srv/www/binning.net/
+          nix shell nixpkgs#rsync --command rsync -av --delete --filter='protect assets/' book/ /srv/www/binning.net/
           printf "✓ Local deployment complete!\n"

.forgejo/workflows/ci.yml

@@ -31,5 +31,5 @@ jobs:
           path: book/
       - name: Deploy
         run: |
-          nix shell nixpkgs#rsync --command rsync -av --delete book/ /srv/www/binning.net/
+          nix shell nixpkgs#rsync --command rsync -av --delete --filter='protect assets/' book/ /srv/www/binning.net/
           printf "✓ Local deployment complete!\n"

.forgejo/workflows/nightly.yml

@@ -36,5 +36,5 @@ jobs:
           path: book/
       - name: Deploy
         run: |
-          nix shell nixpkgs#rsync --command rsync -av --delete book/ /srv/www/binning.net/
+          nix shell nixpkgs#rsync --command rsync -av --delete --filter='protect assets/' book/ /srv/www/binning.net/
           printf "✓ Local deployment complete!\n"