Clear uploads.ts dead code; make lint/typecheck CI gates required

Commit notes

Removes the abandoned chunked-upload code path and the type/lint debt it carried, so the backend is clean under both `biome check src` and `tsc --noEmit` (32 typecheck errors → 0).

uploads.ts: - Remove four disabled chunked-upload endpoints (PUT /chunks/:i, POST /presigned-urls, POST /chunks/:i/confirm, POST /chunks/confirm-batch). Each threw "Chunked uploads are disabled" as its first statement; the unreachable code below produced 29 null-safety errors. The frontend uses none of them (it uploads via the direct presigned flow). - Remove the duplicate "legacy" POST /:uploadId/complete handler (shadowed by the identical active one earlier in the chain). - Drop now-unused storage imports and refresh the stale header doc comment.

sharing-limits.ts: - Widen getSharingTierForPlan/getSharingLimitsForPlan params from the SubscriptionPlan union to string. The implementation already prefix-matches with a free-tier default, so it safely accepts the raw DB plan string; removes 3 errors at the share/upload-link call sites without casts.

Repo-wide Biome formatting fixes (quote/import/wrapping normalization, no behavior change) so `biome check src` passes clean.

CI: drop continue-on-error from the lint and typecheck jobs in ci/ci.yml; update ci/README.md and CLAUDE.md to reflect that both are now blocking gates.

[private session redacted]