bascule-gateway: fix env-prefix separator + embedded accord parse

Two bugs surfaced when bascule-gateway pods first reached Running and
attempted config load (F.4 deployment):

1. Env-var override didn't take effect for any BASCULE_* variable.
   config::Environment::with_prefix("BASCULE") without an explicit
   prefix_separator strips the literal "BASCULE" with no separator,
   so BASCULE_ACCORD_PATH became "_ACCORD_PATH" (leading underscore)
   which doesn't match the field "accord_path". Result: every env
   override silently fell back to the default in config.rs, and the
   pod read /accord/accord.yaml instead of /etc/bascule/accord.yaml
   from the configured volume. Adds .prefix_separator("_") to match
   QM's pattern in services/quartermaster/src/config.rs:150.

2. Embedded fallback accord YAML had `sampled: []` and a stray
   `sampleRate: 1`, but the schema has
   `sampled: Option<SampledConfig {events, sample_rate}>` — empty
   list mis-parses as struct. Result: when accord file lookup failed,
   the .expect("empty accord must parse") panicked, crashing the
   bascule-gateway container. Now omitted (Option default None).

Both fixes verified against accord-core's schema in
services/accord-core/src/schema.rs.

Signed-off-by: Tyler J King <tking@guildhouse.dev>

bascule-gateway/src/config.rs

@@ -121,6 +121,7 @@ impl BasculeConfig {
         let config = config::Config::builder()
             .add_source(
                 config::Environment::with_prefix("BASCULE")
+                    .prefix_separator("_")
                     .separator("__")
                     .try_parsing(true),
             )

bascule-gateway/src/main.rs

@@ -69,8 +69,8 @@ spec:
     fidelity: always_notarize
     notarize: []
     logOnly: []
-    sampled: []
-    sampleRate: 1
+    # sampled omitted — Option<SampledConfig> default None (struct shape:
+    # {events: [...], sample_rate: N}), `sampled: []` would mis-parse.
   reconciliation:
     defaultWindow: "24h"
     onExpiry: alert