[FIX] deltatech_stock_inventory: clear inventory note after applying (18.0)

[dhongu]

Backport of the applicable part of PR #2794 (19.0), found while working helpdesk ticket #9156 (AGROAMAT).

The 19.0 regression that PR fixed — the per-line Inventory Note never reaching the generated stock move — never existed on 18.0: values["name"] = self.inventory_note or values["name"] has always been active here. Verified on the customer's 18.0 production database: an adjustment with a note produces a stock move whose reference is that note.

Two real defects do exist on 18.0, and this PR fixes both:

1. The note is never cleared. action_apply_inventory resets only inventory_id/inventory_line_id, and the standard action_clear_inventory_quantity does not touch custom fields. Confirmed empirically on the customer's production database: after applying, inventory_note was still set on the quant, so it would silently become the reference of the next adjustment on the same product.
2. The column label was untranslated — msgid "Inventory Note" had an empty msgstr, so operators saw it in English.

Adds three tests (32 tests total on the module, 0 failures locally on 18.0). One of them documents a behaviour worth knowing: the user name the standard appends to the move reference comes from user_id (Assigned To), not from the user performing the adjustment — so the per-line note is the only per-product trace of the reason.

README.rst / static/description/index.html are left out of this commit on purpose: they carry uncommitted regeneration from another session in this worktree, and they are generated files.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.54%. Comparing base (2ebbf74) to head (7222246).

Additional details and impacted files

@@            Coverage Diff             @@
##             18.0    #2829      +/-   ##
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+ Coverage   74.50%   74.54%   +0.03%     
==========================================
  Files        1278     1279       +1     
  Lines       22018    22051      +33     
  Branches     2981     2981              
==========================================
+ Hits        16405    16438      +33     
  Misses       4737     4737              
  Partials      876      876              

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