[Refactor]: Source member flows from ntuai.dev MCP

[viiccwen]

Summary

Replace Ian's local and legacy member data paths with ntuai.dev Payload MCP as the single source of truth.

Closes #116

Changes

• add typed User, Membership, tier and platform domain models using MCP website attribute names
• add MemberMcpRepository for validated Payload MCP reads and updates
• add MemberService for membership, binding, subscription, personal prompt and reminder-recipient rules
• migrate Discord, Facebook, LINE, Ian MCP tools, Agent runtime and reminder jobs to MemberService
• support Discord, Facebook and LINE proactive notifications
• enforce subscribe as one of discord, fb or line, with null representing no subscription
• remove local member JSON/cache, legacy member API, synchronization and role fallbacks
• document the migration architecture, security boundaries and operating procedure
• replace legacy member tests with repository, service and integration coverage

Behavior

• tier 0 is non-member or expired
• tiers 1, 2 and 3 represent lecture exploration, hands-on practice and project practice
• MCP failures fail closed for member authorization
• ambiguous user or membership records are rejected instead of selecting the first result
• writes use an allowlist and read-after-write verification
• no local member-data fallback or backward-compatibility layer remains

Validation

• uv run ruff check .
• uv run pytest — 255 passed