feat: bootstrap Alembic and add acronym_mapping table

[borkarsaish65]

Adds Alembic with a no-op baseline revision for the pre-existing unmanaged translations table, plus a migration that creates acronym_mapping (id PK, expansions as JSONB array, description, composite index on acronym+is_active) and seeds it from data/acronyms.csv, now the real 599-acronym dataset.

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