ci: move Linux jobs (backend, web, docker api/web) to self-hosted runner

POS stays on windows-latest (tag/manual only). Runner is registered on
the stage server, systemd-managed, labels [self-hosted, Linux, X64].
Goal: drop dependency on the 2000 GitHub-hosted minute quota — Windows
POS build now runs at most once per release tag.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ concurrency:
 jobs:
   backend:
     name: Backend (.NET 8)
-    runs-on: ubuntu-latest
+    runs-on: [self-hosted, linux]
     services:
       postgres:
         image: postgres:16-alpine
@@ -50,7 +50,7 @@ jobs:
 
   web:
     name: Web (React + Vite)
-    runs-on: ubuntu-latest
+    runs-on: [self-hosted, linux]
     defaults:
       run:
         working-directory: src/food-market.web

.github/workflows/docker.yml

@@ -21,7 +21,7 @@ permissions:
 jobs:
   api:
     name: API image
-    runs-on: ubuntu-latest
+    runs-on: [self-hosted, linux]
     steps:
       - uses: actions/checkout@v4
 
@@ -47,7 +47,7 @@ jobs:
 
   web:
     name: Web image
-    runs-on: ubuntu-latest
+    runs-on: [self-hosted, linux]
     steps:
       - uses: actions/checkout@v4