Remove unwired duplicate-detection feature and Mollie migration columns

Commit notes

Follow-up dead-code removal now that nothing is deployed:

Duplicate detection (server side worked, but no UI ever called it): - Delete POST /uploads/check-duplicates, the frontend calculateFileFingerprint/checkForDuplicates helpers, the worker's quick-fingerprint computation and Redis quickhash cache, the computeQuickFingerprint util + its tests, and the samples.quick_hash column (migration 0030 reverted outright — it only ever existed on this branch). The content hash stays: the analysis worker still uses it to dedup identical files.

Mollie (replaced by Stripe before launch; backend/src was already Mollie-free): - Strip the mollie_* columns and indexes from migrations 0003, 0004 and 0007, and turn 0015 (Mollie Connect onboarding columns) into a kept-for-sequence no-op. The Stripe migration (0021) adds its columns fresh with no reference to the Mollie ones, so fresh databases now come up without stray payment-provider columns.

Checked but intentionally NOT removed: auth/password.ts has no legacy hash fallback — its "legacy" mention is a defensive catch that maps malformed hashes to a failed login instead of a 500.

Co-Authored-By: Claude Fable 5 <[email redacted]> Claude-Session: [private session redacted]