fix(pricing): validate asset prices before storing and guard the DEX reference amount

[Danswar]

Why

The hourly asset-pricing job on dev has been failing on every run for 7+ days:

ERROR [DexService] Error while getting target amount (Arbitrum/TGT -> Arbitrum/USDT): Error: Invariant failed: AMOUNT

The stored approxPriceChf for TGT on dev is a degenerate near-zero value (observed: 4.24e-60). Asset.minimalPriceReferenceAmount inverts the stored price, so the reference amount used for the quote overflows uint256 and the Uniswap SDK rejects it.

Two things make this worse than a single bad quote:

1. The asset is stuck in a self-poisoning loop. The DEX quote is itself the price source for this asset (pricing-dex.service), so the failing quote prevents the corrupt price from ever being overwritten. Prod's TGT row is healthy (0.0061 CHF, refreshing hourly), which confirms the pool quote is fine — only the stored dev value is corrupt.
2. A degenerate stored price leaks everywhere. approxPriceChf also feeds the public asset DTO, custody balance values, trading profit estimates, and the persisted daily price history — so it must not be stored in the first place.

What

• Asset.isSanePrice() with sanity rails (1e-12–1e15): a price outside these is corrupted data, not a real price.
• Read side (minimalPriceReferenceAmount): falls back to a reference amount of 1 for insane prices instead of inverting them. This breaks the deadlock: the next pricing run can quote again and overwrite the corrupt value.
• Write side (AssetPricesJobService.updatePrices): a degenerate fetched price is no longer stored — the job logs a warning and keeps the last known good price, protecting all downstream consumers at the source.

Replaces the asset-price half of #4200, which only guarded the single read site and had no write-path validation.

Test plan

• New asset.entity.spec.ts (sanity rails + reference-amount fallback) and asset-prices-job.service.spec.ts (stores sane prices, skips degenerate ones, keeps last known good, continues with remaining assets) — 9/9 passing
• Full local suite green (pre-push hook)
• Lint / tsc --noEmit / prettier clean
• After dev deploy: verify the hourly DexService error stops and TGT's price self-heals