fix(coturn): remove plaintext TURN shared secret, rotate leaked value

Issue #38 discussion surfaced a real bug: the TURN shared secret was
duplicated - correctly SOPS-encrypted in coturn-secret.yaml, but also
hardcoded in plaintext in synapse-values.yaml (a plain, non-SOPS
ConfigMap), visible in git history. Also found turn_user_lifetime is
86400000ms (24h), not "short-lived" as previously assumed - raising the
stakes of the leak somewhat.

Extracted the turn config block into its own dedicated SOPS-encrypted
Secret (synapse-turn-secret.yaml), wired via a second HelmRelease
valuesFrom entry (same pattern already used for ess-mas-values-secret).
Rotated the value while doing this, so the leaked plaintext secret is no
longer live anywhere.

Added checksum/rotated-at annotations (matrix-stack HelmRelease's
existing element-config-checksum patch gets a sibling turn-secret-checksum;
coturn's Deployment pod template gets a rotated-at annotation) so future
rotations actually restart both consumers - Kubernetes doesn't restart
running pods when a referenced Secret's content changes on its own.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

apps/production/custom-configs/synapse-values.yaml

@@ -58,15 +58,6 @@ data:
             room_list_publication_rules:
               - user_id: "*"
                 action: allow
-        turn:
-          config: |
-            turn_uris:
-              - "turn:turn.axion1337.chat?transport=udp"
-              - "turn:turn.axion1337.chat?transport=tcp"
-              - "turns:turn.axion1337.chat?transport=tcp"
-            turn_shared_secret: "cab3c8408363515d9b4cdc3384a1f76ca17a973242fdfdc72b67ac4d86158527"
-            turn_user_lifetime: 86400000
-            turn_allow_guests: false
         oembed:
           config: |
             oembed_enabled: true