secrets: add gitea_hermes_webhook_secret to mars + jupiter

Shared HMAC secret for the Gitea -> Hermes webhook relay merged in #2. Both
hosts need the same value: jupiter signs deliveries with it (the webhook
config registered by gitea-hermes-webhook-provision), and mars verifies them
in the relay and hands it to Hermes through the hermes-agent.env template.

32 random bytes, hex-encoded, stored with no trailing newline — the value
reaches Hermes via an env-file template where a newline would both corrupt
the file and change the key the HMAC is computed with.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01S94o42aQ8VkBmEWvDem5xa

secrets/jupiter.yaml

@@ -14,6 +14,7 @@ sabnzbd_web_password: ENC[AES256_GCM,data:9Lo=,iv:H0Kz8A534RxX+7/Aue8Q87gCzSY5e/
 sabnzbd_nzb_key: ENC[AES256_GCM,data:DNVenqhJ7wf5Ng0XRA1gJN95e+90e6D9NImOSHJv/Us=,iv:eqFn0stB5pqh0ls4/impD8gc/lOkORwEJzRP6m7u1XU=,tag:Zs8ogLBZEZLyMvFBqhfpIA==,type:str]
 sabnzbd_eweka_username: ENC[AES256_GCM,data:eLsTZoM8T8fAlGaXWlDaoQ==,iv:eawyGhN7+d6UfBIbI3y1qgq+MYBGrXP6VfAkSOK6llA=,tag:ELOfQGHU5NOxZFhKOKf8LA==,type:str]
 sabnzbd_eweka_password: ENC[AES256_GCM,data:Mt3ZHAe2wzacCQq3x9Uy8WxjrVNad1SmU6sl8ZgrkMLymfq2eP4JzO/uPdD33A==,iv:PnFT95Zxqz4QBpPF5PRloKpoa15AU7Ef/Owwy+iDotw=,tag:/uRX00RzHLJN3gws5Qz8SA==,type:str]
+gitea_hermes_webhook_secret: ENC[AES256_GCM,data:Q8e+mj05MJI7CEJwRonpOmQphAZ0CfnZFoGxrDSSiyHoH3BNhqBU5gBmzuu+6NK9OS33kN+JnFvrwCeEzVxooA==,iv:mdsKOMD5B0Jzh1YRmRh71P8Io9RFtI6aqAky5x+WxOQ=,tag:v3LKz5a8ayl7WAzIPbwj6Q==,type:str]
 sops:
     age:
         - enc: |
@@ -34,7 +35,7 @@ sops:
             CzjSDQZTcseEXZNwuzZcfB5Mvq0BQvjOj7lGuxzuE4qwWkdJWGfVLQ==
             -----END AGE ENCRYPTED FILE-----
           recipient: age1zak7glavmg4026p2389fyqe769vqm4jrryknuqckgqq4merz5f7q44rkkt
-    lastmodified: "2026-08-21T23:14:15Z"
-    mac: ENC[AES256_GCM,data:7ts5oWyiPAUtF8OokDqzjZnoH0CCRwdsvF330SeCBnoyATXsWsXOvrLdTVBJf24MSMyJxCQSqBUpzKVVIlVOL1C4KjA+axr34M4oWJ/kEUReO1q9Lrl3/SuuV8PLji6/Z7pTU9tuhl4jsIPdzDsM9oZv6PbxXeex/d4fiw8Qex4=,iv:KX/xBM7HZ2NoCt4T8dhYA7o6h2eBAOdsegEplbxIAnM=,tag:KyddDKzAXV6jvMjnEV0H2Q==,type:str]
+    lastmodified: "2026-08-23T03:16:52Z"
+    mac: ENC[AES256_GCM,data:uQcOxORIWugK43LpQLI7JEjH6oGooseKCQQt0d+n43i7o23JGdUN5Wy/iD7GqmtVVZod02gl1ohEXV+kpvgetFpAO5NZu76HUVPFgaLOx+2LjrR1pNpC+52Iqlx52uypwby9eDvnC01jLFHu2l13NGBrLM3JQGmEXF57phzM/Q4=,iv:H7o3gdx/1GmZ1FRm7z97TNmiVpm6YFCEk0Puw4ZETDs=,tag:bsz3bcK2z3szrwpo55bSzQ==,type:str]
     unencrypted_suffix: _unencrypted
     version: 3.13.3