fix(server): persist inventory changes outside the close event

[Qwerty1Verified]

Summary

Inventory persistence had three gaps:

• Inventories only reached the database on the close event (server/main.lua:210).
• The txAdmin shutdown handler wrote only inventories that were currently open, so
  anything mutated without being opened and closed was never persisted at all.
• onResourceStop did no saving, so a restart qb-inventory discarded in-memory
  state for every inventory, including open ones.

This closes all three:

• New SaveInventoryItems(identifier) helper holds the upsert; the shutdown and close
  handlers now call it instead of carrying their own copies of the SQL. This consolidates writes to one helper.
• AddItem and RemoveItem write immediately when the target is an Inventories
  entry that is not open, on the basis that an open inventory will get a close event
  and be saved then; the at-risk mutations are exactly the ones to closed inventories.
• onResourceStop flushes open inventories when qb-inventory itself stops.

The resulting invariant: a closed inventory is always current in the database, and an
open one is written on close, on shutdown, or on resource stop. Only a hard crash
mid-session loses anything. Both existing handlers keep their isOpen conditions,
which are now sufficient rather than gaps.

Related issue

No issue. Found while working on #647; a headless AddItem to a stash nobody had opened succeeded in memory with no persistence.

Change type

• Bug fix
• New feature
• Breaking change
• Documentation
• Refactor, maintenance, or performance improvement

Testing

• FXServer artifact: b35265
• qb-core version or commit: 1.3.0
• Resource version or commit: 2.2.1 / 2ff8fb2
• Server operating system: Windows Server 2022
• Test steps and results:

1. Headless AddItem to a closed stash, then restart qb-inventory with no open or
   close in between; item present afterwards. Previously lost.
2. Item dragged into a stash left open, then restart qb-inventory; item present.
   Previously lost. This exercises the onResourceStop flush
3. Dropped an item, dragged another into the bag, restarted, then checked
   SELECT identifier FROM inventories WHERE identifier LIKE 'drop-%'; empty, so
   drops remain memory-only and no phantom rows are created.
4. Open, add, close, restart; item present, close path unchanged.
5. /giveitem and a relog; player inventories unaffected.

Compatibility and migration

None

Checklist

• I agree to follow the QBCore FiveM Code of Conduct.
• My pull request title follows Conventional Commits, such as fix(scope): ... or feat(scope): ....
• This pull request contains one focused change and does not include unrelated formatting or refactoring.
• I tested the change on a current FXServer artifact with the relevant official resources and dependencies.
• Existing repository checks and linting pass.
• I documented new behavior and identified every breaking or migration-related change.
• I added or updated configuration examples, SQL migrations, and translations where applicable.
• I removed credentials, webhook URLs, license keys, database data, player identifiers, and other private information.
• I have the right to submit all included code and assets under the repository's license.
• I understand and reviewed all submitted code, including any AI-assisted code, and accept responsibility for its correctness and licensing.