feat(usage): persist pricing basis and takeover dimensions in storage (schema v11)

- proxy_request_logs: add pricing_model column recording the basis actually
  used at write time (NULL = pre-v11 rows, '' = unpriced error rows)
- cost backfill recomputes strictly by the persisted basis; the
  request_model fallback now only applies to placeholder models, so
  real-but-unpriced takeover rows stay at zero cost until pricing is
  added instead of being permanently frozen at the alias's price
- backfill_missing_usage_costs_for_model can locate rows by pricing_model
- usage_daily_rollups: rebuild with request_model + pricing_model in the
  primary key so the alias-to-real-model mapping and the pricing basis
  survive the 30-day prune; legacy rows migrate with ''
- rollup_and_prune backfills costs before pruning: prune is irreversible
  and used to run before the startup backfill, permanently booking
  then-unpriced rows as zero
- get_model_stats groups by the effective pricing model
  (COALESCE(NULLIF(pricing_model,''), model)) so costs aggregate under
  the model whose prices produced them; response-mode behavior unchanged

src-tauri/src/database/mod.rs

@@ -49,7 +49,7 @@ use std::sync::Mutex;
 
 /// 当前 Schema 版本号
 /// 每次修改表结构时递增，并在 schema.rs 中添加相应的迁移逻辑
-pub(crate) const SCHEMA_VERSION: i32 = 10;
+pub(crate) const SCHEMA_VERSION: i32 = 11;
 
 /// 安全地序列化 JSON，避免 unwrap panic
 pub(crate) fn to_json_string<T: Serialize>(value: &T) -> Result<String, AppError> {