Add PremiumService, normalize PremiumModel, refactor API/Cog, add tests and docs, bump discord.py

[bergaman9]

Motivation

• Consolidate premium subscription logic into an application service to separate business rules from API and bot cogs.
• Normalize tier/payment representations and make activation idempotent to reduce payment/transaction edge cases.
• Provide a runnable migration/refactor plan and documentation for production-grade premium/billing changes.

Description

• Added a new PremiumService (src/services/premium_service.py) that centralizes get_status and activate_with_payment use-cases and enforces idempotency and payment verification.
• Updated the premium data model (src/database/models/premium.py) to normalize tier and payment method values and adjusted in-place activation/upsert behavior.
• Refactored API routes (src/api/routes/premium_api.py) to use PremiumService and simplified route logic to delegate verification/activation to the service; preserved a TODO for proper JWT auth.
• Refactored the Discord cog (src/cogs/premium/premium.py) to use PremiumService, normalize displayed tiers, and fixed permission checks to use app_commands.checks.has_permissions.
• Bumped discord.py minimal version in requirements.txt from >=2.3.0 to >=2.5.2.
• Added design and rollout documentation (docs/PRODUCTION_REFACTOR_AND_PREMIUM_PLAN.md, docs/REFACTOR_PLAN_DISCORDPY_PREMIUM.md) describing architecture, migration, and sprint roadmap.
• Added unit/integration-style tests for the service (tests/test_premium_service.py) using lightweight dummy model/crypto service fakes.

Testing

• Added tests/test_premium_service.py covering success path, idempotency, and unknown-tier handling for activate_with_payment and executed the suite with pytest.
• Test run: all tests in tests/test_premium_service.py passed (3 tests, async pytest markers).
• No other automated CI gates were modified in this change.

---

Codex Task