fix: escape literal $ in .env values before envsubst (Pattern B stacks)

Root cause of the LITELLM_MASTER_KEY/LITELLM_SALT_KEY truncation
incident (2026-08-26): stack-deploy.sh's single-file deploy path is
`envsubst "$VARS" < stack.yaml | docker stack deploy -c - stack`.
envsubst embeds the raw secret value into the compose YAML text. If
that value contains a literal '$' followed by word chars, the
resulting YAML now contains what looks like a second variable
reference. `docker stack deploy -c -` runs Compose's own interpolation
pass on that text before creating the service, finds no such env var,
and silently substitutes empty string -- truncating the secret in the
running container with no error.

Confirmed: an 87-char LITELLM_MASTER_KEY arrived in the ai_litellm
container as 73 chars, silently, on a real deploy.

This is not specific to ai -- it affects every Pattern B stack (host
.env + envsubst, not native Docker secrets): maintenance, media,
unifi, guacamole, security, auth, traefik, meshcentral, ddm. Any of
them could have a '$'-containing value truncating right now without
detection, since the failure produces no warning.

Fix: escape every literal '$' as '$$' in export/export_merged (which
feed the `eval` that sets envsubst's actual source values), before
envsubst ever sees them. envsubst does not interpret '$' in replacement
text, so the doubled dollar survives envsubst intact; Compose's own
interpolation pass then consumes exactly one level of escaping,
landing on the correct single '$' with no leftover false variable
reference. vars/vars_merged (envsubst's allowlist string, unrelated to
values) are untouched.

NOT deployed/merged yet -- pending review. The currently-running
ai_litellm service still has the truncated keys and needs a fresh
`stack-deploy.sh ai` run after this merges to pick up the corrected
values.

ai/ai.yaml

@@ -1,3 +1,8 @@
+# NOTE: litellm's AWS/DB/master-key secrets, open-webui's secret key/DB URL,
+# and the OAuth client secret are provisioned into ai/ai.env at deploy time
+# from Woodpecker secrets (see PRs #4, #6, #7) -- not committed here.
+# MCPO_API_KEY (used by mcpo and mcpo-critical) is NOT yet migrated; it
+# remains manually managed in ai/ai.env by design (see PR #4 discussion).
 services:
   open-webui:
     image: ghcr.io/open-webui/open-webui:0.11.1

deploy/envparse.py

@@ -22,14 +22,58 @@ def merge_envs(base_path, override_path):
     merged = {**base, **override}
     return list(merged.items())
 
+# ─────────────────────────────────────────────────────────────────────────────
+# 2026-08-26 FIX — double-interpolation truncation bug (Pattern B stacks):
+#
+#   stack-deploy.sh's single-file/no-extras path is:
+#     envsubst "$VARS" < ai.yaml | docker stack deploy -c - ai
+#
+#   envsubst substitutes ${VAR} placeholders in the compose YAML with the
+#   literal, raw value of each shell-exported variable. If that raw value
+#   itself contains a literal '$' followed by word characters (e.g. a
+#   randomly-generated secret like "...i*Edu$RyAVYTqr4yzSS##..."), the
+#   resulting YAML text now contains what LOOKS like a second variable
+#   reference. `docker stack deploy -c -` runs Compose's own interpolation
+#   pass on that YAML text before creating the service — and Compose sees
+#   that leftover "$RyAVYTqr4yzSS", finds no such env var, and silently
+#   substitutes empty string. The secret gets truncated in the running
+#   container with NO error or warning.
+#
+#   Confirmed impact (2026-08-26): LITELLM_MASTER_KEY and LITELLM_SALT_KEY
+#   in the `ai` stack were both truncated at their first literal '$' after
+#   a real deploy — 87-char secret arrived in the container as 73 chars.
+#
+#   This affects every stack using Pattern B (host .env + envsubst, not
+#   native Docker secrets): ai, maintenance, media, unifi, guacamole,
+#   security, auth, traefik, meshcentral, ddm — any of them could have a
+#   '$'-containing value silently truncating right now without detection,
+#   since the failure is silent and only visible by diffing the source
+#   value against the live container env.
+#
+#   Fix: escape every literal '$' in a value as '$$' at export time, BEFORE
+#   envsubst ever sees it. envsubst does not interpret '$' in the
+#   replacement text (only in the template), so the doubled dollar survives
+#   envsubst untouched. Compose's interpolation pass then consumes exactly
+#   one level of escaping ('$$' -> literal '$'), landing on the correct
+#   original single '$' with no leftover variable-reference lookalike.
+#
+#   Only applied in export/export_merged (which feed `eval` to set the
+#   actual values envsubst reads) — NOT in vars/vars_merged, which just
+#   build envsubst's space-separated $VARNAME allowlist string and have
+#   nothing to do with actual values.
+# ─────────────────────────────────────────────────────────────────────────────
+
+def escape_dollar(v):
+    return v.replace('$', '$$')
+
 mode = sys.argv[1]
 if mode == 'export':
     for k, v in parse_env(sys.argv[2]):
-        print('export {}={}'.format(k, repr(v)))
+        print('export {}={}'.format(k, repr(escape_dollar(v))))
 elif mode == 'export_merged':
     # export_merged <global.env> <stack.env>
     for k, v in merge_envs(sys.argv[2], sys.argv[3]):
-        print('export {}={}'.format(k, repr(v)))
+        print('export {}={}'.format(k, repr(escape_dollar(v))))
 elif mode == 'vars':
     print(' '.join('$' + k for k, v in parse_env(sys.argv[2])))
 elif mode == 'vars_merged':