Fix user profile confidence decay

[x-Spartacus]

Closes #115

• Normalize profile data so generated preferences get lastUpdated
• Apply confidence decay before user profile learning merges existing profile data
• Make profile refresh run confidence decay immediately and report whether decay was applied
• Keep decay idempotent so repeated refreshes do not double-decay the same elapsed time
• Add tests for generated timestamps, decay, and removal below the confidence floor

Tested:

• npx bun test tests/profile-write.test.ts
• npx bun run typecheck
• npx prettier --check src/services/user-profile/user-profile-manager.ts src/services/user-memory-learning.ts src/services/api-handlers.ts tests/profile-write.test.ts
• npx bun run build
• npx bun test

Full test result: 155 pass, 0 fail.