fix: harden CID validation, apply artisan fee tiers correctly, and migrate platform config storage

[testersweb0-bug]

Description

This PR addresses one security vulnerability and two logic issues related to IPFS CID validation, fee calculation, and storage safety.

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Key Changes

Hardened IPFS CIDv1 Validation (#172)

• Strengthened validation logic to prevent malformed CID inputs
• Added:
  • Length constraints for common multibase encodings (Base32, Base16, Base58)
  • Version prefix validation (e.g., ba for Base32 CIDv1)

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Respect Artisan-Specific Fee Tiers (#170, #169)

• Fixed issue where artisan-specific fee tiers were ignored in payout flows
• Refactored all payout logic to use get_effective_fee_bps
• Ensures correct fee application across:
  • Partial refunds
  • Dispute resolutions
  • General payouts

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Storage Key Migration

• Migrated platform configuration to structured storage:
  • DataKey::PlatformConfig
• Prevents key collisions and improves maintainability

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Verification Results

Automated Tests

• test_validate_ipfs_cid_v1_stricter: PASS

• test_validate_ipfs_cid_v1_too_short: PASS

• test_validate_ipfs_cid_v1_wrong_version: PASS

• test_accept_partial_refund_with_custom_fee_tier: PASS

• test_platform_config_sanity_check: PASS

• All baseline tests: PASS

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Related Issues

Closes #170
Closes #172
Closes #169

[Agbeleshe]

LGTM!!!!