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Summary

Adds the product user-stories document docs/user-stories.md — the core product-design reference for future UI/backend implementation.

This addresses the host rant 2026-08-14T20:04:34 ("编写产品用户故事（User Stories）— 从不同用户角色视角描述使用流程").

Related Issue

N/A (host instruction rant)

Changes

• Role definitions (Goals / Pain points) for 5 roles: Visitor/New User, Regular User (public), Sharer, Enterprise Admin, Enterprise Member
• 21 user stories in standard format (As a … / I want … / so that …) with Acceptance Criteria, covering the core loops (register → get points → consume; list key → consumed → earn; enterprise configure → allocate → use) and key operations (marketplace search/filter/sort, sharing pause/delete, transactions, API Key CRUD+copy, member top-up, department management, usage reports)
• 4 key user journeys with mermaid diagrams (public: register → first consume, share → first earning; enterprise: admin configure → employees use, employee consume → request top-up)
• 12 edge/exception scenarios (key invalid/revoked, quota exhausted, insufficient points, department deleted → members unassigned, API key leak, etc.)
• Alignment table with ui/ prototype and docs/architecture.md

Tests

• cargo test passes (1 passed)
• cargo fmt --check passes
• Markdown sanity check: balanced code fences, all 5 roles present, 4 journeys, 12 edge cases
• New unit tests added (N/A — docs-only change)