fix(i18n): completes Vietnamese parity, fixes empty migration query

Two more release/v3.8.50 base-red items, both surfaced while chasing
CI failures on unrelated PRs:

- vi.json was missing 8 keys that #9539 (NewAPI/Sub2API aggregator
  balance) added to en.json without a matching i18n:sync-ui run —
  pt-BR.json already had all 8, only Vietnamese drifted. Added
  translations for the 6 provider-settings strings, the feature-flag
  description, and the quota tooltip; verified against
  tests/unit/i18n-vi-completeness.test.ts (parity, placeholder
  preservation, ICU parse — all 5 assertions pass).

- src/lib/db/migrations/120_interception_rules.sql was pure comments
  documenting a no-schema-change key_value namespace, with no
  executable SQL statement — the migration runner logged
  "FAILED: 120_interception_rules — Query contained no valid SQL
  statement" on every fresh DB init. 118_provider_param_filters.sql
  (same pattern, two migrations earlier) already ends with a bare
  `SELECT 1;` no-op for exactly this reason; 120 was just missing it.
  Verified directly against better-sqlite3 that the file now executes
  without error.

src/lib/db/migrations/120_interception_rules.sql

@@ -14,3 +14,4 @@
 -- falls back to the existing native web-search-bypass defaults in webSearchFallback.ts).
 --
 -- See: src/lib/db/interceptionRules.ts
+SELECT 1;