Add alpha_numeric widget for nullable numeric fields with special code support

[Copilot]

Odoo float fields cannot hold NULL — the sentinel hack (-0.01) works for blank display but breaks down when scores need to carry semantic meaning like "Absent" or "Cheating". This adds a proper solution: a paired Char field (score_alpha) that holds either a formatted number or a configurable special code, keeping the underlying float field in sync automatically.

Widget (alpha_numeric)

New OWL field widget registered as "alpha_numeric" for char fields:

• Accepts numbers or configurable special codes (default: ["-", "A", "C"])
• Formats numbers on display via decimal_places + use_separator options
• Auto-derives the paired numeric field from the _alpha suffix convention (score_alpha → score); overridable via numeric_field option
• Writes both fields atomically on commit — special codes write sentinel to the float, numbers write the parsed value
• Falls back to displaying the numeric field's value for pre-existing records (backward compat)
• Inline validation error; Enter commits, Escape cancels; works in editable list and form views

<field name="score_alpha"
       widget="alpha_numeric"
       options="{'allowed_codes': ['-', 'A', 'C'],
                 'decimal_places': 2,
                 'use_separator': false}"/>

Python model (aps.resource.submission)

• score_alpha and out_of_marks_alpha Char fields added (both tracking=True)
• Module-level alpha_to_float(val, sentinel) helper — parses number strings, returns sentinel for empty/special codes
• @api.onchange handlers keep the numeric fields live during UI edits
• write() override syncs alpha → numeric on programmatic saves (runs before the auto_score flag check so the flag is correctly set to False)

Migration (18.0.1.0.45)

Post-migration back-fills score_alpha / out_of_marks_alpha from existing float values using FM format with 6 decimal places to preserve precision.