fix(gateway): close billing, limit, routing and translation gaps

Billing:
- Passthrough SSE streams now bill the usage the upstream reports.
  The gateway falls back to a request-size estimate only on a 2xx
  stream or a truncated JSON body.
- A unified stream that fails after it starts is estimated, not
  billed at zero.
- Billing uses the built-in LiteLLM price table and honours
  modelMeta free: true.
- Pricing entries are validated. Cost arithmetic rounds the total and
  saturates on overflow.
- Gemini thinking tokens count as completion tokens.

Limits and storage:
- A rejected request rolls back its counts on the other scopes, so one
  user cannot lock out a group.
- Redis EXPIRE replies are checked. Keys recreated without a TTL are
  repaired at once.
- A mutating pipeline is not retried after reply bytes arrive.
- History, model totals and target usage report a store outage.

Routing and translation:
- Discovered ids with a provider-like prefix are listed only in
  owner/id form, so listing and routing agree.
- Anthropic and Gemini discovery follows pagination.
- The developer role maps to system.
- stream_options.include_usage is honoured on translated streams.
- tool_choice none keeps tools when history holds tool blocks.
- refusal maps to content_filter. Data URI parameters are stripped.

Streaming, retry and failover:
- SSE accepts CR, LF and CRLF line ends. This closes a CR injection
  path.
- Retry-After is never jittered. A cancelled wait returns the
  context error.
- The watchdog timer stops at end of body.
- Oversized bodies return 413.
- A failed stream ends with an error event.
- Failover restores pre-existing headers. Client cancels do not count
  as provider successes.

Also: "admin" is a reserved provider name. metrics.path must start
with "/". Pricing warnings go to the right gateway instance. The admin
targets view sends access as null or [] so the two states differ.
Upstream organisation headers are stripped. The users-file reload
error is logged once per error. Dead code is removed, and test-only
helpers move to test files.

Claude-Session: https://claude.ai/code/session_01V5jYJNxpDFdJkyyExpADrv

target_health.go

@@ -216,24 +216,13 @@ func sanitizeTargetErr(msg, rawURL string) string {
 	return sanitizeProviderErr(msg, rawURL)
 }
 
-// stateOf reports (kind, name)'s current targetHealthState — see this
-// type's own doc comment for the exact unknown/healthy/unhealthy rule.
-func (t *targetHealthTracker) stateOf(kind, name string) targetHealthState {
-	if t == nil {
-		return targetHealthUnknown
-	}
-	t.mu.Lock()
-	defer t.mu.Unlock()
-	e, ok := t.entries[targetHealthKey{kind: kind, name: name}]
-	if !ok {
-		return targetHealthUnknown
-	}
-	return t.stateForLocked(e)
-}
-
 // stateForLocked derives e's targetHealthState. Caller must hold t.mu.
-// Split out of stateOf/snapshot so both apply the identical rule. The
-// threshold check runs first and always wins (F4): a target that has
+// Split out so every reader applies the identical rule: production reads
+// go through snapshot (below), and the test-only stateOf helper (finding
+// 18 fix, review-routes.md — moved to target_health_test.go, since it
+// has zero production call sites; snapshot already covers every real
+// reader) shares this same derivation rather than a second copy of it.
+// The threshold check runs first and always wins (F4): a target that has
 // failed failureThreshold times straight reads unhealthy whether or not
 // it has ever succeeded. Below threshold, a target that has never once
 // succeeded reads unknown rather than healthy — see targetHealthUnknown's