P2-1-T1. Migration 0228 adds the 1:1 document_content_index sibling of
inbound_documents: AES-256-GCM ciphertext of the extracted text plus a
deduped HMAC-SHA256 token array (search_tokens) with a GIN index for the
array-overlap match the list search will union with title/filename ILIKE.
Both foreign keys cascade. Nothing readable sits at rest; the index subkey
is HKDF-derived at runtime, so tokens are opaque without the server key.
