refactor(db): drop six verified-dead repository/bundle methods (vulture, #1136)

[axisrow]

Part of #1136 (vulture axis of epic #1130). Six methods deleted; each verified dead across all five surfaces (FastAPI/Web/CLI/agent/TUI) before removal. Per #1136, "dead by graph" ≠ "unneeded" — so every candidate passed a full grep + git-history check first.

For each: repo-wide grep (src/ tests/ scripts/ docs/ conftest.py) returned exactly 1 hit — the definition — with no string references. The codebase's dynamic repo access targets other specific names (clear_dialogs, get_recent_for_channels, get_channels, …), and the dispatcher's dynamic dispatch is f"_handle_{command}"-shaped and unrelated to any of these.

Repository methods (unused readers; the repo layer stays)

1. ChannelsRepository.get_channels_by_tag — born in feat: RSS, tags, S3 storage, refinement pipeline, ReAct fallback, test fixes #271 (tags feature) with no caller (git grep at 2af529e: definition only). The tag feature's surfaces use list_all_tags/create_tag/delete_tag/set_channel_tags/get_channel_tags (see src/cli/commands/channel.py); this reverse reader ("channels by tag") was never wired to any surface.
2. DialogCacheRepository.has_dialogs — born in bc2bddd (persist dialog cache) with no caller. Live dialog_cache methods (enumerated from call sites): clear_all_dialogs, clear_dialogs, count_dialogs, get_all_phones, get_cached_at, get_dialog, list_dialogs, replace_dialogs — has_dialogs absent.
3. GenerationRunsRepository.list_by_status — definition-only at every commit that touched it (feat: add 12 missing agent tools and fix ghost MODULE_GROUPS entries #292, [codex] close issue 554 audit gaps #555, feat(parity): close CLI ↔ Web ↔ Agent-tool parity gaps #780); moderation/runs surfaces use list_pending_moderation / list_runs instead.

Bundle wrappers (unused thin wrappers over LIVE repo methods that stay)

4. AccountBundle.list_live_usable_accounts — AccountsRepository.get_live_usable_accounts stays (17 live refs).
5. AccountBundle.update_premium — AccountsRepository.update_account_premium stays (24 live refs).
6. MessageBundle.get_message_stats — MessagesRepository.get_stats stays (used via Database.get_stats → dashboard route).

No public-API/parity impact: none of the six is reachable from any of the five surfaces.

Verification (no regression guard possible — code is gone; proof = green suite + this note)

• ruff check src/ tests/ conftest.py — clean.
• Import smoke test for all four touched modules — OK.
• pytest tests/repositories/ tests/test_bundles.py — 426 passed.
• Full suite (pytest tests/ -m "not aiosqlite_serial" -n auto then -m aiosqlite_serial) — green. The only failures elsewhere are pre-existing on origin/main: tests/test_quality_scoring_service.py (7 tests fail on clean main) and the xdist-flaky tests/routes/test_debug_routes.py::test_debug_timing_page; both deselected and reproduced on clean main, unrelated to this diff.

Not deleted here (doubtful → left for owner decision)

• "Test-only" code in the DB layer is absent from this PR; bundle wrappers like update_flood, save_stats, list_notification_queries are called only by their own unit tests and are deliberately left untouched pending owner call (test-locking vs delete-with-tests). They are flagged in the issue audit, not here.

Part of #1136 / #1130.