refactor(db): dedupe Z.AI migrations, account row mapping, emoji query (#1135)

[axisrow]

Part of #1135 (axis 5 of #1130). Three clones from the #1135 target list, all genuine duplication (no parity surface involved).

What & how

migrations.py — twin Z.AI base_url rewriters

_migrate_zai_legacy_base_url and _migrate_zai_empty_base_url_to_coding shared an identical load-JSON → walk zai items → save-on-change body (21-line clone, 109 tokens). Extracted _rewrite_zai_base_urls(db, *, should_rewrite, new_base_url, log_message). Each public migration is now a thin caller passing its own predicate + target URL.

• Both still run only when something changed (unchanged).
• Each keeps its own provider_registry import + distinct log message.

accounts.py — duplicated Account row mapping + forked SELECT

• The Account(...) constructor block was verbatim in get_accounts and get_live_usable_accounts. Extracted _account_from_row(row, session_string); the two methods differ only in decrypt-failure handling (raise vs skip), which stays in the caller.
• get_decrypted_session reimplemented the SELECT-by-id/phone + decrypt. It now delegates to get_session_export, so the row-binding consistency guard (REST: POST /api/accounts/{id}/export-session #1145) — phone and session bound to the same row read — is no longer forked. Returns exported[1] (the session).

messages.py — get_trending_emojis

Two near-identical SQL bodies differing only by a collected_at window fragment. Collapsed into one query with an optional WHERE fragment + parameterized params list.

Verification

• tests/test_migrations.py + tests/repositories/ (400) green.
• Accounts / session-export / trending-emojis keyword tests (481 passed, 6 skipped) green.

jscpd

All three clones from the #1135 target list (migrations.py:720↔762, accounts.py two pairs, messages.py) are removed.

Part of #1135 / #1130. Not for merge — review requested.

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