feat: implement user-level storage quota

[jack-wang-176]

feat: implement user-level storage quota

Description

This PR introduces a user-level storage quota mechanism within the existing tenant-level quota framework.
It enables Tenant Owners and Admins to assign specific storage limits to individual users, providing granular control over resource consumption. The system now enforces a dual-tier quota check (Tenant-level and User-level) on operations that consume storage, such as document uploads and FAQ indexing.

Key additions:

• Database & Model: Added storage_quota and storage_used fields to tenant_members table with corresponding migrations.
• Backend API: Added PUT /api/v1/tenants/:id/members/:user_id/quota endpoint for Admins to adjust user quotas.
• Service Layer: Updated all storage adjusting/checking points (file upload, chunk processing, deleting, FAQ indexing) to atomically account for both tenant and user storage.
• Frontend: Added a visual edit flow in the "Tenant Members" table to display used/quota and allow Admins to configure individual limits via a dialog.

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📚 Documentation update
• 🎨 Refactor
• ⚡ Performance improvement
• 🧪 Test
• 🔧 Configuration / Build / CI

Related Issue

Fixes #1653

Testing

• Local Database Migration: Verified SQL up/down scripts apply cleanly.
• UI Editing Flow: Created a new account, granted Admin rights, and modified another member's tenant storage quota via the new UI dialog. Verified the payload via browser network requests and confirmed the successful update in the backend terminal logs.
• Upload Interception: Temporarily set a user's quota to 10 bytes and verified that attempting to upload a document correctly blocks the operation and surfaces the new USER_STORAGE_QUOTA_EXCEEDED error.
• Accounting Validation: Uploaded and deleted documents and monitored both tenant-level and user-level storage_used fields to ensure they atomically increment/decrement in sync.

Checklist

• make fmt && make lint && make test pass locally
• Self-reviewed the code
• Added/updated tests covering the change
• Updated related documentation (README, docs/, Swagger annotations, etc.)
• Breaking changes are clearly called out in the description above

Screenshots / Recordings

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