fix: prevent duplicate pending operations in mod queue

[cebarks]

Summary

• Adds partial unique indexes to enforce one pending operation per (mod/addon, action)
• Prevents duplicate queue entries from concurrent requests
• Returns user-friendly error messages for duplicates

Changes

1. Migration 019: Creates partial unique indexes on pending_operations:

   • idx_pending_ops_mod_action on (forge_mod_id, action) for mod operations
   • idx_pending_ops_addon_action on (forge_addon_id, action) for addon operations

2. Database layer (src/db/users.rs):

   • insert_pending_op: Returns friendly error message for UNIQUE constraint violations
   • has_pending_url_op: New helper to check for duplicate URL-based operations

3. Web handler (src/web/handlers/mods.rs):

   • URL install path checks for duplicates before downloading
   • Gracefully handles constraint violations with user-facing info messages

Test plan

• Verified compilation with just check
• Verified no clippy warnings with just clippy
• Manual test: queue same mod install twice (should show "already queued" message)
• Manual test: queue same URL install twice (should show "already queued" message)
• Manual test: concurrent queue requests (database constraint should catch race)

Root Cause

The pending_operations table had no unique constraint on (forge_mod_id, action) or (forge_addon_id, action), allowing duplicate pending operations to be queued.

Implemented with the help of Claude Code