fix(proxy): use parse-time {$SHARE_KEY} in share_gate instead of runtime placeholder

{env.SHARE_KEY} is not resolved by Caddy's query matcher or CEL expression at
request time — the literal placeholder string was compared against the URL key,
causing every external request to fall through to 403. {$SHARE_KEY} is
substituted by the Caddyfile adapter at reload/start, baking the literal key
value into the compiled config.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Caddyfile

@@ -36,22 +36,27 @@
 	remote_ip 192.168.1.0/24 172.16.0.0/12 100.64.0.0/10 127.0.0.1
 }
 
-# Shared-link gate — external users visit ?k={$SHARE_KEY} once to get a 30-day cookie.
+# Shared-link gate — external users visit ?k=<SHARE_KEY> once to get a 30-day cookie.
 # Internal IPs and tailnet clients pass through unconditionally.
 # Usage: import share_gate  (replaces import internal_only on shared services)
+# Uses {$SHARE_KEY} (parse-time substitution) — baked in at caddy reload/start when the
+# env var is set. Rotate: update SHARE_KEY in stack.env, push → Portainer redeploys → key changes.
 (share_gate) {
+	@external not remote_ip 192.168.1.0/24 172.16.0.0/12 100.64.0.0/10 127.0.0.1
+	handle @external {
 		route {
 			@has_key query k={$SHARE_KEY}
 			handle @has_key {
 				header Set-Cookie "share_access={$SHARE_KEY}; Path=/; Max-Age=2592000; HttpOnly; Secure; SameSite=Strict"
 				redir {http.request.uri.path} 302
 			}
-
-		@deny_external {
-			not remote_ip 192.168.1.0/24 172.16.0.0/12 100.64.0.0/10 127.0.0.1
-			not expression `{http.request.cookie.share_access} == "{$SHARE_KEY}"`
+			@has_cookie expression `{http.request.cookie.share_access} == "{$SHARE_KEY}"`
+			handle @has_cookie {
+			}
+			handle {
+				respond "Access denied" 403
+			}
 		}
-		respond @deny_external "Access denied" 403
 	}
 }