feat: WebSearch tri-state, account stats pricing fix, quota cache fix, usage tooltip

WebSearch tri-state switch:
- Account-level web_search_emulation changed from bool to tri-state
  string: "default" (follow channel) / "enabled" / "disabled"
- shouldEmulateWebSearch checks channel config when account is "default"
- SQL migration converts old bool values
- Frontend select replaces toggle in Edit/CreateAccountModal

Account stats pricing:
- resolveAccountStatsCost uses upstream model (post-mapping) for matching
- Priority: custom rules → model pricing file (when toggle on) → default
- Custom rules always configurable, independent of toggle
- Account ID field changed to searchable selector filtered by platform
- Description updated to reflect new behavior

Quota notification cache fix:
- CheckAccountQuotaAfterIncrement fetches real-time account from DB
- Reconstructs pre-increment usage for accurate threshold crossing detection
- New AccountQuotaReader interface (minimal: GetByID only)

Usage tooltip:
- Per-request/image billing shows per-request price instead of $0 token price
- Token billing continues to show input/output price per million tokens

backend/migrations/105_migrate_websearch_emulation_to_tristate.sql

@@ -0,0 +1,11 @@
+-- Convert old boolean web_search_emulation to tri-state string
+-- true → "enabled", false → remove key (becomes "default")
+UPDATE accounts
+SET extra = (extra - 'web_search_emulation') || jsonb_build_object('web_search_emulation', 'enabled')
+WHERE extra ? 'web_search_emulation'
+  AND extra->>'web_search_emulation' = 'true';
+
+UPDATE accounts
+SET extra = extra - 'web_search_emulation'
+WHERE extra ? 'web_search_emulation'
+  AND extra->>'web_search_emulation' = 'false';