feat(channels): add custom account stats pricing rules

Allow channels to configure independent model pricing for account
statistics cost calculation, decoupled from user billing.

Backend:
- Migration 101: channels.apply_pricing_to_account_stats toggle,
  channel_account_stats_pricing_rules/model_pricing tables,
  usage_logs.account_stats_cost column
- resolveAccountStatsCost: match rules by group/account, then channel
  pricing, fallback to original formula when unconfigured
- Integrate into both GatewayService.recordUsageCore and
  OpenAIGatewayService.RecordUsage
- Update 8 account stats SQL queries to use
  COALESCE(account_stats_cost, total_cost) * account_rate_multiplier
- 23 unit tests for matching, pricing lookup, and cost calculation

Frontend:
- Channel edit dialog: toggle + custom rules UI with group/account
  multi-select and pricing entry cards
- API types and i18n (zh/en)

backend/migrations/101_add_account_stats_pricing.sql

@@ -0,0 +1,38 @@
+-- Account statistics pricing: allow channels to configure custom pricing for account cost tracking.
+
+-- 1. Channel-level toggle
+ALTER TABLE channels ADD COLUMN IF NOT EXISTS apply_pricing_to_account_stats BOOLEAN NOT NULL DEFAULT FALSE;
+
+-- 2. Account stats pricing rules (ordered list per channel)
+CREATE TABLE IF NOT EXISTS channel_account_stats_pricing_rules (
+    id BIGSERIAL PRIMARY KEY,
+    channel_id BIGINT NOT NULL REFERENCES channels(id) ON DELETE CASCADE,
+    name VARCHAR(100) NOT NULL DEFAULT '',
+    group_ids BIGINT[] NOT NULL DEFAULT '{}',
+    account_ids BIGINT[] NOT NULL DEFAULT '{}',
+    sort_order INT NOT NULL DEFAULT 0,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+CREATE INDEX IF NOT EXISTS idx_cas_pricing_rules_channel_id ON channel_account_stats_pricing_rules(channel_id);
+
+-- 3. Model pricing for each rule (same structure as channel_model_pricing)
+CREATE TABLE IF NOT EXISTS channel_account_stats_model_pricing (
+    id BIGSERIAL PRIMARY KEY,
+    rule_id BIGINT NOT NULL REFERENCES channel_account_stats_pricing_rules(id) ON DELETE CASCADE,
+    platform VARCHAR(50) NOT NULL DEFAULT '',
+    models JSONB NOT NULL DEFAULT '[]',
+    billing_mode VARCHAR(20) NOT NULL DEFAULT 'token',
+    input_price NUMERIC(20,10),
+    output_price NUMERIC(20,10),
+    cache_write_price NUMERIC(20,10),
+    cache_read_price NUMERIC(20,10),
+    image_output_price NUMERIC(20,10),
+    per_request_price NUMERIC(20,10),
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+CREATE INDEX IF NOT EXISTS idx_cas_model_pricing_rule_id ON channel_account_stats_model_pricing(rule_id);
+
+-- 4. Usage logs: pre-computed account stats cost (NULL = use default formula)
+ALTER TABLE usage_logs ADD COLUMN IF NOT EXISTS account_stats_cost NUMERIC(20,10);