ci: trust Nexus mirror CA in backend dotnet restore (fixes skipped deploys)

The mirror's Let's Encrypt cert renewed under the new ISRG Root YR root,
which isn't in the dotnet SDK image's trust store. `dotnet restore` validates
TLS and fails (NU1301 / unable to get local issuer certificate), so both
backend CI jobs fail and the deploy is skipped. The npm jobs are unaffected
because they already pass --strict-ssl=false.

Pin the mirror's intermediate (CN=YR2, CA:TRUE, valid to Sept 2028) and add it
as a trust anchor before restore in:
- CI api-build + admin-api-build jobs (.gitea/workflows/ci-cd.yml)
- docker/api/Dockerfile + docker/admin-api/Dockerfile (deploy image builds)

Also set NUGET_CERT_REVOCATION_MODE=offline in the CI restore steps to avoid
CRL/OCSP fetches to lencr.org (filtered from Iran).

Permanent fix is server-side (re-chain to ISRG Root X1 or update trust stores);
this unblocks CI/deploys without depending on that.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitea/workflows/ci-cd.yml

@@ -80,10 +80,20 @@ jobs:
           </configuration>
           EOF
 
+      - name: Trust Nexus mirror CA
+        # The mirror's Let's Encrypt cert renewed under the new ISRG Root YR, which is
+        # not yet in the SDK image's trust store. The npm jobs skip TLS via
+        # --strict-ssl=false; dotnet validates, so add the mirror's intermediate
+        # (CA:TRUE, valid to Sept 2028) as a trust anchor.
+        run: |
+          cp docker/nexus-mirror-ca.crt /usr/local/share/ca-certificates/nexus-mirror-ca.crt
+          update-ca-certificates
+
       - name: Restore
         run: dotnet restore src/Meezi.API/Meezi.API.csproj --configfile /tmp/nuget.ci.config
         env:
           DOTNET_CLI_TELEMETRY_OPTOUT: 1
+          NUGET_CERT_REVOCATION_MODE: offline
 
       - name: Build
         run: dotnet build src/Meezi.API/Meezi.API.csproj --no-restore -c Release
@@ -128,10 +138,18 @@ jobs:
           </configuration>
           EOF
 
+      - name: Trust Nexus mirror CA
+        # See api-build: trust the mirror's intermediate so dotnet restore validates
+        # the new ISRG Root YR chain (npm jobs sidestep this with --strict-ssl=false).
+        run: |
+          cp docker/nexus-mirror-ca.crt /usr/local/share/ca-certificates/nexus-mirror-ca.crt
+          update-ca-certificates
+
       - name: Restore
         run: dotnet restore src/Meezi.Admin.API/Meezi.Admin.API.csproj --configfile /tmp/nuget.ci.config
         env:
           DOTNET_CLI_TELEMETRY_OPTOUT: 1
+          NUGET_CERT_REVOCATION_MODE: offline
 
       - name: Build
         run: dotnet build src/Meezi.Admin.API/Meezi.Admin.API.csproj --no-restore -c Release