Add receipt scanning with Tabscanner OCR

[TobogganDev]

Summary

• Implements the receipt-scan feature end-to-end: POST /receipt now returns the scanned data (was always null), fixes a crashing String.format bug, and raises the multipart upload limit (was capped at Spring's default 1MB).
• Switches the OCR provider from Asprise (no self-serve signup, shared demo key with no usable quota) to Tabscanner (free tier, self-serve signup), mapped into the existing Receipt DTO so no mobile-side contract changes were needed.

Test plan

• Verified POST /receipt returns a populated Receipt body for a real receipt photo, with correct merchant/total/date
• Verified the low-confidence and insufficient-funds rejection paths still work
• Verified the app boots cleanly with TABSCANNER_API_KEY unset (safe fallback, no crash)