Plan and monitor Google Ads in the account's own currency

[demegire]

What and why

Google Ads reads every amount in the account's own currency (CampaignBudget). Launch and monitor treated those amounts as dollars, so a $20 plan became ₺20 a day in a lira account. This replaces #400, which would have required USD accounts, with support for other currencies.

• Launch reads the account's currency. For anything other than USD, it reads the day's ECB reference rate once and saves it as gather:fx. A failed read is retryable and writes nothing.
• The plan works out budget, click ceiling and cost-per-customer targets in dollars as before, then converts each once onto Google's minimum unit (whole yen, won, rupiah; cents elsewhere). The $1 budget and $0.05 ceiling floors still apply at the converted rate. PLAN.md and the approval summary show the account-currency amount, its dollar equivalent and the rate. The rate is pinned on the plan and in campaign.json and never converted again.
• Supported currencies are those Google Ads bills in and the ECB publishes (28). Any other, or a missing currency, gates as unsupported_currency before any rate read, model call or write.
• Request builders check the unit, the converted floors and a runaway cap of $1M equivalent per currency.
• The monitor reads the account's currency each run. Reports, rationales and proposals use it, and proposals carry the currency and rate. The _usd field names on Google Ads amounts are renamed, since they never were dollars.
• Compatibility: plans and proposals saved before this change read as USD, which they were. apply refuses a plan whose currency differs from the account's. A dollar-planned campaign in a non-USD account gets a currency_mismatch alert, plain cost reporting and no cost-based pauses or proposals.

Verification

• New tests/test_paid_ads_currency.py covers:
  • cross rates through the euro and feed failures;
  • rounding and floors;
  • JPY and IDR bundles, with refusals for off-unit or below-floor amounts;
  • a EUR launch end to end with one rate read;
  • unsupported and missing currencies stopping before any rate, model or write;
  • a rate outage that retries;
  • currency-mismatch refusal at apply, and a legacy plan still applying in dollars;
  • monitor proposals in EUR and whole-yen steps, and the mismatch alert.
• Existing paid ads, Google Ads and proposal tests were updated for the renamed fields. Legacy campaign.json and proposal fixtures were kept as back-compat coverage.
• Full suite locally: 4,602 passed, 1,257 skipped (DB-backed, no local Postgres; CI covers them). Ruff, format and import-linter pass.
• A live ECB read through ads_currency converted $20 to €17.89, £15.11, ¥3,172 and ₹1,916.

Notes

• No live Google Ads account was used. One real non-USD launch is still needed before calling this done.
• Launch inputs (daily_budget_usd, cpc_ceiling_usd) stay in USD and are converted, with both amounts shown before approval. The assessment still reasons in USD.
• Launch and monitor policy versions are bumped to 2. A monitor run in progress during the deploy may fail once on renamed fields; the next run is unaffected.
• The ECB read is a fixed public GET from the worker. It needs no credentials and costs nothing.

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