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-tauri/src/proxy/usage/logger.rs

@@ -2,11 +2,11 @@
 
 use super::calculator::{CostBreakdown, CostCalculator, ModelPricing};
 use super::parser::TokenUsage;
-use crate::database::Database;
+use crate::database::{Database, PRICING_SOURCE_REQUEST, PRICING_SOURCE_RESPONSE};
 use crate::error::AppError;
 use crate::services::usage_stats::{find_model_pricing_row, is_placeholder_pricing_model};
 use rust_decimal::Decimal;
-use std::{str::FromStr, time::SystemTime};
+use std::str::FromStr;
 
 /// 请求日志
 #[derive(Debug, Clone)]
@@ -64,13 +64,7 @@ impl<'a> UsageLogger<'a> {
                 )
             };
 
-        let created_at = SystemTime::now()
-            .duration_since(SystemTime::UNIX_EPOCH)
-            .map(|d| d.as_secs() as i64)
-            .unwrap_or_else(|e| {
-                log::warn!("SystemTime is before UNIX_EPOCH, falling back to 0: {e}");
-                0
-            });
+        let created_at = chrono::Utc::now().timestamp();
 
         conn.execute(
             "INSERT OR REPLACE INTO proxy_request_logs (
@@ -227,18 +221,19 @@ impl<'a> UsageLogger<'a> {
             Ok(value) => value,
             Err(e) => {
                 log::warn!("[USG-003] 获取默认计费模式失败 (app_type={app_type}): {e}");
-                "response".to_string()
+                PRICING_SOURCE_RESPONSE.to_string()
             }
         };
-        let default_pricing_source =
-            if matches!(default_pricing_source_raw.as_str(), "response" | "request") {
-                default_pricing_source_raw
-            } else {
-                log::warn!(
+        let default_pricing_source = if default_pricing_source_raw == PRICING_SOURCE_RESPONSE
+            || default_pricing_source_raw == PRICING_SOURCE_REQUEST
+        {
+            default_pricing_source_raw
+        } else {
+            log::warn!(
                 "[USG-003] 默认计费模式无效 (app_type={app_type}): {default_pricing_source_raw}"
             );
-                "response".to_string()
-            };
+            PRICING_SOURCE_RESPONSE.to_string()
+        };
 
         let provider = self
             .db
@@ -271,7 +266,9 @@ impl<'a> UsageLogger<'a> {
         };
 
         let pricing_model_source = match provider_pricing_source {
-            Some(value) if matches!(value, "response" | "request") => value.to_string(),
+            Some(value) if value == PRICING_SOURCE_RESPONSE || value == PRICING_SOURCE_REQUEST => {
+                value.to_string()
+            }
             Some(value) => {
                 log::warn!("[USG-003] 供应商计费模式无效 (provider_id={provider_id}): {value}");
                 default_pricing_source.clone()