feat(storage): move runtime state to SQLite

[owainlewis]

Summary

Moves channel cursors and channel-qualified backend session mappings into the canonical push.db SQLite database. Existing state.json files migrate once through an atomic, idempotent transaction, remain untouched as a recovery copy, and continue to honor explicit state_path, legacy last_row_id, and unqualified iMessage session keys.

The merged PushPaths owner now supplies both the canonical database and legacy migration paths to the state store. Explicit state_path and database_path overrides remain compatible. The Slack durable inbox remains separate on PushPaths.inbox because its acknowledgement-queue lifecycle differs from canonical runtime state. Database and migration errors fail closed before polling or pause processing with actionable context.

Why

Whole-file JSON rewrites created a second mutable source of truth and could not provide database-level concurrency guarantees. SQLite now provides monotonic cursor upserts, transactional session rotation, and crash-safe migration markers under the repository's canonical runtime-path owner.

Test plan

Verified after rebasing onto current main (795f326, including PRs #171, #172, and #175):

• cargo fmt --all -- --check
• cargo clippy --locked --all-targets -- -D warnings
• cargo build --locked
• cargo test --locked (368 unit, 2 docs, 14 CLI integration, 3 crash tests)
• python -m mkdocs build --strict
• Fresh independent review of exact commit 11fb11d: approved with no blocker or important finding

Risks

The main risk is upgrade migration behavior. Tests cover legacy iMessage keys, explicit and fallback iMessage cursors, multiple channels and backends, empty session IDs, migration interruption and repetition, corrupt JSON, database-open failure, concurrent cursor writes, backend changes, exact channel-qualified /clear rotation, and runtime path overrides. The retained legacy JSON is intentionally not deleted or rewritten after migration.

Related issue

Closes #170