Add lint:check, remove unused variables, const in e2e spec

[NteinPrecious]

Closes #1401
Closes #1403
Closes #1404

Also covers #1402 only partially (not closed): the Prettier formatting pass (prettier --write over src and test) was not done because prettier is not installed and dependencies were not to be installed. #1402 stays open for that.

• package.json / README: added lint:check (same glob, no --fix) and documented both lint scripts
• [Backend] Remove the 7 unused variables reported by ESLint #1403: removed the unused ipfsService, result, stopTimer, noop, tx and the dead GIST_POSTED_EVENT; configured the underscore-prefix pattern for no-unused-vars in eslint.config.mjs (covers _sinceBlock)
• [Backend] Use const for ipfsMock and sorobanMock in gists.e2e-spec.ts #1404: ipfsMock and sorobanMock are now const in test/gists.e2e-spec.ts

Tests and lint were not run.

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