Migrate upstream mirrors to mirrors/ Forgejo org (#265)

## Summary

- Created `mirrors` Forgejo organization for upstream mirror repos
- Transferred 22 mirror repos from `eblume/` to `mirrors/` (mirror sync config preserved)
- Deleted unused repos: hajimari, hister
- Updated all container build URLs (homepage, navidrome, ntfy Dockerfiles + nix)
- Updated documentation references (migrate-forgejo-from-brew, upstream-fork-strategy, fix-ntfy-nix-version)
- `dotfiles` intentionally kept under `eblume/` per user request
- `devpi` transferred to `mirrors/`

Repos remaining under `eblume/`: blumeops, cv, mcquack, dotfiles

## Cleanup TODO

- [ ] Delete temp Forgejo API token "claude-migration-temp" (Settings > Applications)

## Test Plan

- [x] Verified mirror config (mirror=true, original_url) survived transfer on test repo (tesla_auth)
- [x] All pre-commit hooks pass (including container-version-check, docs-check-links)
- [ ] Verify a mirror repo sync runs successfully after transfer (check mirrors/authentik or similar)
- [ ] Rebuild containers from branch to verify Dockerfile URLs resolve

Reviewed-on: https://forge.ops.eblu.me/eblume/blumeops/pulls/265

containers/ntfy/Dockerfile

@@ -12,7 +12,7 @@ RUN apk add --no-cache git
 
 RUN mkdir /app && cd /app \
     && git init \
-    && git remote add origin https://forge.ops.eblu.me/eblume/ntfy.git \
+    && git remote add origin https://forge.ops.eblu.me/mirrors/ntfy.git \
     && git fetch --depth 1 origin ${NTFY_COMMIT} \
     && git checkout FETCH_HEAD
 
@@ -28,7 +28,7 @@ RUN apk add --no-cache build-base git
 
 RUN mkdir /app && cd /app \
     && git init \
-    && git remote add origin https://forge.ops.eblu.me/eblume/ntfy.git \
+    && git remote add origin https://forge.ops.eblu.me/mirrors/ntfy.git \
     && git fetch --depth 1 origin ${NTFY_COMMIT} \
     && git checkout FETCH_HEAD