fix: IPv6 address truncation and image proxy SSRF vulnerabilities

Replace strings.Split(RemoteAddr, ":") with net.SplitHostPort for correct
IPv6 address extraction in vote deduplication and comment IP tracking.

Harden image proxy: add SSRF-safe transport blocking private/reserved IPs
at connection time with DNS rebinding protection, sanitize error messages
to prevent information leakage, add response size limit via io.LimitReader.

Fix shadowed error variables in BlockedUsers, SetTitle, and Delete methods.
Exclude gosec taint analysis false positives at linter config level.

.github/workflows/ci-backend.yml

@@ -60,13 +60,13 @@ jobs:
       - name: golangci-lint
         uses: golangci/golangci-lint-action@v9
         with:
-          version: "v2.6.0"
+          version: "v2.10.1"
           working-directory: backend/app
 
       - name: golangci-lint on example directory
         uses: golangci/golangci-lint-action@v9
         with:
-          version: "v2.6.0"
+          version: "v2.10.1"
           args: --config ../../.golangci.yml
           working-directory: backend/_example/memory_store