fix(usage): pricing routing, SSE lifecycle, and validation hardening

* model pricing routing: extend prefix-match families (gpt-/o1-o5/
  gemini-/deepseek-/qwen-/glm-/kimi-/minimax-) with per-family dash
  thresholds so short base IDs like gpt-5 no longer mis-match
  gpt-5-mini; strip ISO and 8-digit date suffixes via UTF-8-safe
  byte matching so claude-haiku-4-5-20251001 falls back to
  claude-haiku-4-5 pricing
* SSE collector: SseUsageFinishGuard (RAII) guarantees finish() on
  early return or panic; AtomicBool fast path lets push() skip the
  Mutex once first-event time is recorded
* validation: shared validate_cost_multiplier / validate_pricing_source
  helpers across DAO and service layers; PRICING_SOURCE_RESPONSE /
  PRICING_SOURCE_REQUEST constants replace string literals; price
  fields in update_model_pricing now reject empty / non-decimal /
  negative input before INSERT
* backfill: add backfill_missing_usage_costs_for_model so a single
  price edit only scans matching rows instead of the full log table;
  startup backfill remains full-scan
* session_usage{,_codex,_gemini}: share find_model_pricing helper from
  usage_stats; metadata_modified_nanos centralizes mtime precision
* frontend: NON_NEGATIVE_DECIMAL_REGEX + isNonNegativeDecimalString
  replace three copies of the same multiplier regex; isUnpricedUsage
  surfaces zero-cost rows that have usage tokens (cached per row to
  avoid double evaluation); invalidate usageKeys.all on pricing mutate
  so backfilled rows refresh

src/components/usage/PricingConfigPanel.tsx

@@ -28,7 +28,7 @@ import {
 } from "@/components/ui/select";
 import { useModelPricing, useDeleteModelPricing } from "@/lib/query/usage";
 import { PricingEditModal } from "./PricingEditModal";
-import type { ModelPricing } from "@/types/usage";
+import { isNonNegativeDecimalString, type ModelPricing } from "@/types/usage";
 import { Plus, Pencil, Trash2, Loader2 } from "lucide-react";
 import { toast } from "sonner";
 import { proxyApi } from "@/lib/api/proxy";
@@ -143,7 +143,7 @@ export function PricingConfigPanel() {
         );
         return;
       }
-      if (!/^-?\d+(?:\.\d+)?$/.test(trimmed)) {
+      if (!isNonNegativeDecimalString(trimmed)) {
         toast.error(
           `${t(`apps.${app}`)}: ${t("settings.globalProxy.defaultCostMultiplierInvalid")}`,
         );
@@ -281,6 +281,7 @@ export function PricingConfigPanel() {
                       <Input
                         type="number"
                         step="0.01"
+                        min="0"
                         inputMode="decimal"
                         value={appConfigs[app].multiplier}
                         onChange={(e) =>