feat: V2 pack create/update endpoints with trail_id support

[Angak0k]

Context

PR #214 introduced the trails database table and admin management endpoints. During this transition period, V1 pack create/update endpoints still accept trail names (strings) and resolve them to trail_id internally.

Scope

Create V2 pack create/update endpoints that accept trail_id (integer) directly instead of trail name strings:

• POST /api/v2/mypack - create pack with trail_id field
• PUT /api/v2/mypack/:id - update pack with trail_id field
• Admin equivalents under /api/admin

Additional cleanup (once V2 is stable)

• Remove the legacy trail TEXT column from the pack table
• Remove dual-write logic from V1 handlers
• Remove V1 trail name resolution code

References

• Issue feat: Classify trails by country/continent and move to database #187 (original trails database feature)
• PR feat: trails database & admin management (#187) #214 (trails database implementation)