Optimize MDI metadata file (88% smaller) and add weekly optimizer workflow

[Copilot]

Ports the MDI metadata optimization from argon_industria_oled. The original materialdesignicons-webfont_meta.json (~3.16 MB) carried per-icon UUIDs, tags, authors, versions and empty alias arrays — none of which are used at render time. Flattening to a name/alias → codepoint map shrinks it to ~354 KB (88.8%) and replaces the O(n) scans in icons.py with a single dict lookup.

Changes

• scripts/optimize_materialdesignicons_meta.py — flattens the metadata, sorted, written compact (no whitespace). Idempotent on already-optimized input so the workflow is safe to re-run.
• materialdesignicons-webfont.meta.json — new optimized asset; old materialdesignicons-webfont_meta.json removed. New file name per the problem statement.
• custom_components/opendisplay/imagegen/icons.py — draw_icon and draw_icon_sequence now do mdi_data.get(icon_name) against the flat dict. The old list-of-objects format is no longer supported (no backward-compat path requested).
• .github/workflows/optimize-mdi-metadata.yaml — weekly (Mon 00:00 UTC) + workflow_dispatch. Runs the optimizer, and if the file changed, closes any prior automated PR for the same base branch and opens a new one with before/after size stats. Branch name is sanitized for refs containing /.

Lookup before/after

# before: linear scan over ~7.3k entries, then a second scan over aliases
for icon in mdi_data:
    if icon['name'] == icon_name:
        chr_hex = icon['codepoint']; break
if not chr_hex:
    for icon in mdi_data:
        if 'aliases' in icon and icon_name in icon['aliases']:
            chr_hex = icon['codepoint']; break

# after: single hash lookup, aliases pre-merged at build time
chr_hex = mdi_data.get(icon_name)