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

provider_openai_test.go

@@ -934,7 +934,9 @@ func TestValidateConfigName(t *testing.T) {
 		{"provider", "v1", true},          // reserved: unified API namespace
 		{"provider", "mcp", true},         // reserved: MCP target-proxy prefix
 		{"provider", "a2a", true},         // reserved: A2A target-proxy prefix
+		{"provider", "admin", true},       // reserved: admin dashboard's own fixed routes (review-auth.md F9)
 		{"mcpServers", "v1", true},        // same reserved set applies to every kind
+		{"mcpServers", "admin", true},
 		{"agents", "a2a", true},
 		{"provider", ".", true},    // dot-only: matches configNamePattern's character class but rejected anyway
 		{"provider", "..", true},   // dot-only: same
@@ -962,6 +964,7 @@ func TestBuildAdapters_RejectsReservedOrInvalidProviderName(t *testing.T) {
 		{"v1"},
 		{"mcp"},
 		{"a2a"},
+		{"admin"},
 		{"has a space"},
 	}
 	for _, tt := range tests {