refactor: migrate reputation score to per-user persistent storage

[EmeditWeb]

Problem

All reputation scores are stored in a single instance storage Map<Address, u32> (storage.rs lines 27-46). Every score read/write loads/saves the entire Map. For large user bases this will exceed Soroban instance storage size limits.

Contract impact: At scale, the reputation contract will hit instance storage caps and fail to read/write scores for new users.

Before Starting

Read: context/architecture-context.md

Root Cause

Instance storage was used for simplicity during initial implementation.

What To Build

1. Migrate to per-user persistent storage:
   • Key: DataKey::Score(address)
   • Value: u32 score
   • TTL extension on every write
2. Keep get_score() interface unchanged
3. Write a migration test verifying data integrity
4. Document the storage layout change

Files To Touch

• contracts/reputation-contract/src/storage.rs — rewrite read_score/write_score
• contracts/reputation-contract/src/types.rs — add DataKey variant
• contracts/reputation-contract/src/tests.rs — add migration tests

Acceptance Criteria

• Scores stored per-user in persistent storage
• TTL extended on every score write
• get_score returns same values as before
• All existing tests pass
• Scaling test with 1000+ users passes

Mandatory Checks

• context/ files read
• cargo build passes
• cargo test passes
• PR references this issue