chore(security): bump idna >=3.15 (CVE-2026-45409) + ignore disputed PyJWT advisory

- requirements.txt: pin idna>=3.15 to clear ReDoS in idna.encode() on
    crafted Unicode payloads. Transitive via anyio/httpx/requests/yarl,
    so the explicit floor stops a future downstream loosening from
    silently downgrading us.
  - security.yml: permanently --ignore-vuln CVE-2025-45768 (PyJWT). The
    advisory is disputed by the maintainers — "key length is chosen by
    the application" — and no fix version exists. Bambuddy is safe:
    auto-generates secrets via secrets.token_urlsafe(64) and rejects
    file-loaded secrets shorter than 32 chars (auth.py:177, :184).
  - security.yml: drop the stale Pygments --ignore-vuln CVE-2026-4539.
    Pygments has been patched upstream; the ignore no longer matches
    anything.

.github/workflows/security.yml

@@ -132,10 +132,17 @@ jobs:
       - name: Run pip-audit
         id: pip-audit
         run: |
-          # CVE-2026-4539: low-severity ReDoS in Pygments AdlLexer (indirect dep via mkdocs-material/pytest/rich).
-          # No fix available yet. Remove --ignore-vuln once Pygments releases a patched version.
-          pip-audit --desc on --format json --output pip-audit-results.json --ignore-vuln CVE-2026-4539 || echo "vulnerabilities_found=true" >> $GITHUB_OUTPUT
-          pip-audit --desc on --ignore-vuln CVE-2026-4539 || true
+          # CVE-2025-45768 (PYSEC-2025-183 / GHSA-65pc-fj4g-8rjx): disputed by PyJWT maintainers.
+          # Advisory says "key length is chosen by the application that uses the library" — no
+          # PyJWT fix exists or will exist. Bambuddy is safe: backend/app/core/auth.py:184 uses
+          # secrets.token_urlsafe(64) (~86 chars of entropy) for auto-generated secrets and
+          # rejects file-loaded secrets shorter than 32 chars at :177. Keep ignored permanently.
+          pip-audit --desc on --format json --output pip-audit-results.json \
+            --ignore-vuln CVE-2025-45768 \
+            || echo "vulnerabilities_found=true" >> $GITHUB_OUTPUT
+          pip-audit --desc on \
+            --ignore-vuln CVE-2025-45768 \
+            || true
 
       - name: Upload audit results
         if: always()