ci: add path filtering to skip unnecessary CI runs (#111)

test.yml: Use dorny/paths-filter to detect changed paths. Backend
tests only run when backend/**, biome.json, or the workflow itself
changes. Frontend build only runs when frontend/**, biome.json, or
the workflow changes. Jobs skipped via job-level 'if:' are treated
as passed by GitHub required checks.

codeql.yml: Only run on push/PR when JS/TS source files, package
files, or CodeQL config changes. Weekly schedule and manual dispatch
remain unfiltered.

.github/workflows/codeql.yml

@@ -3,8 +3,30 @@ name: "CodeQL"
 on:
   push:
     branches: [main]
+    paths:
+      - '**.js'
+      - '**.ts'
+      - '**.tsx'
+      - '**.jsx'
+      - 'backend/package.json'
+      - 'backend/package-lock.json'
+      - 'frontend/package.json'
+      - 'frontend/package-lock.json'
+      - '.github/codeql/**'
+      - '.github/workflows/codeql.yml'
   pull_request:
     branches: [main]
+    paths:
+      - '**.js'
+      - '**.ts'
+      - '**.tsx'
+      - '**.jsx'
+      - 'backend/package.json'
+      - 'backend/package-lock.json'
+      - 'frontend/package.json'
+      - 'frontend/package-lock.json'
+      - '.github/codeql/**'
+      - '.github/workflows/codeql.yml'
   schedule:
     - cron: "0 6 * * 1" # Weekly on Monday at 6am UTC
   workflow_dispatch: # Allow manual trigger