Filter inactive gists, key-less Soroban reads, drop dead cache, scope winston lint rule

[LGTM-ButItsBroken]

Closes #1405
Closes #1406
Closes #1407
Closes #1408

• winston.config.ts: kept import = require (the project has no esModuleInterop, and enabling it would change every import * as of a callable CJS module) with a narrowly scoped eslint-disable and an explanatory comment
• gists.service.ts: removed the dead CacheEntry, contentCache and CACHE_TTL_MS
• gist.repository.ts: findNearby, findByGistId, countNearby and countNearbyByCell now require is_active = true; findByStellarGistId is left as is since the indexer uses it. Added integration tests that flip is_active and assert the gist disappears
• soroban.service.ts: the four live reads (getGistLive, isActiveLive, listGistsByCellLive, getAdminLive) take their simulation source from a new optional SOROBAN_READ_ACCOUNT (validated as a Stellar public key), falling back to the signer; they no longer need STELLAR_SECRET_KEY

.env.example is not touched here to avoid conflicting with the env-doc PR; SOROBAN_READ_ACCOUNT is validated in env.validation.ts. Tests and build were not run.

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