refactor(db): dedupe cross-bundle wrappers into shared mixins (#1135)

[axisrow]

Part of #1135 (axis 5 of #1130). Stays off the parity surface — only genuine within-repo duplication is touched.

What

src/database/bundles.py duplicated identical repository pass-through methods across bundles:

• channel read/state ops — ChannelBundle + CollectionBundle
• task create / task read — both bundles
• get_setting/set_setting — four bundles (Collection, Notification, Search, Scheduler)
• account reads (list_accounts, list_account_summaries) — three bundles
• message search_messages/insert_messages_batch — Collection + Search
• get_recent_searches — Search + Scheduler

jscpd flagged 5 clone pairs inside the file — the top cluster of #1135.

How

Each duplicated wrapper group moves into a repo-scoped plain-class mixin (_ChannelOps, _CollectionTaskCreateOps, _CollectionTaskReadOps, _SettingsOps, _NotificationQueriesOps, _MessageOps, _AccountReadOps, _SearchLogReadOps). A bundle mixes in only the groups whose repository field it already carries.

Mixins are deliberately NOT dataclasses: their attribute annotations (channels: ChannelsRepository, …) do not become dataclass fields, so every bundle's positional constructor order is unchanged.

Verification

• Public API byte-identical — verified by an inspect.signature diff of all 10 bundle classes before/after. The only diffs are PipelineBundle.list_accounts/list_account_summaries, which gain the return-type annotations the duplicates already carried.
• Bundle method count 138 → 111; the test_db_access_conventions ratchet baseline is lowered accordingly (it is a downward-only cap by design).
• tests/test_bundles.py (42) + bundle consumers (scheduler, search_queries, notifier, photo_loader, pipeline_service, regression-services: 178) green.

jscpd

• Before (this file): 5 internal clones (~80 dup lines)
• After: 0 internal clones; the two remaining cross-file hits (bundles.py:19 ↔ facade.py:18 import block; bundles.py:734 ↔ photo_loader.py:456 repo pass-through) are import-boilerplate / single thin wrappers, not worth a helper.

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

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