Migration V26: Rename checksum fields and add per-user deduplication.

This migration renames checksum fields for clarity and adds per-user file deduplication while preserving the ability to query for popular files across all users.

Changes

• Enables pgcrypto extension for cryptographic functions
• Renames checksum column to file_checksum (for clarity)
• Drops unique index on file_checksum (allows same file from different users)
• Adds user_file_checksum column (SHA256 of user_id + file_checksum)
• Creates unique index on user_file_checksum to enforce per-user uniqueness
• Backfills existing records with calculated user_file_checksum values

Requirements

• PostgreSQL with pgcrypto extension support (enabled automatically)

Purpose

• Same user cannot upload the same file twice (duplicate prevention via user_file_checksum)
• Different users CAN upload the same file (no unique constraint on file_checksum)
• Original file_checksum field preserved for finding most popular images across all users
• Clearer naming: file_checksum vs user_file_checksum