Read nested Notion blocks in page order

[Floating-Y]

Why

Notion's children endpoint returns one level at a time. The connector only read top-level blocks, omitting paragraphs inside toggles and columns. This fixes a traversal gap in the original connector, not a regression from the rate-limit fix in #269.

What changes

• Read nested blocks in parent/child/sibling order, with separate pagination cursors for each parent.
• Share one Paced client and one MAX_BLOCKS_PER_PAGE budget across the entire page, retaining request spacing, 429 retries, and Retry-After handling. Stop making requests when the budget is exhausted.
• Traverse containers without renderable text while preserving child_page and child_database boundaries.
• Preserve Markdown rendering, page identity, title, last-edited metadata, and ingestion. Child-request errors propagate to the existing warning and title-only fallback; this does not change the synchronization error policy.
• Add wiremock regressions against the production traversal and an isolated PostgreSQL resync test. The API address seam is private; there are no new dependencies or user-facing settings.

How it was checked

• Before the traversal fix: the regression run had 9 failures. After the fix: all 18 Notion tests passed, with database execution required.
• Coverage includes toggles, columns, multiple nesting levels and sibling order, independent top-level/child pagination, containers without text, the shared block limit with no extra requests, independent content boundaries, child-request failures, pacing/429 retries, and unchanged flat-page output.
• Resync with mocked Notion responses and real isolated PostgreSQL: the same external key and unchanged last-edited timestamp update an existing top-level-only document with the missing body, create one new version, and produce Unchanged on the next identical sync.
• Live Notion synthetic fixture: production body fetching and page search matched all 10 expected text lines exactly across 13 blocks, including nested toggles, columns, sibling order, identity, and metadata. A dedicated live acceptance test passed (1 passed, 0 ignored). This validates API reads; resync through the running application against real Notion remains unverified.
• cargo fmt --all --check and cargo clippy --locked --workspace --all-targets -- -D warnings passed.
• The initial parallel workspace test run hit two existing chat-registry deadline failures. All five chat-registry tests passed on serial retry, and the complete workspace run with --test-threads=1 passed with database and PDF checks required. The existing real-workspace smoke test was filtered out of that run; the dedicated live test above was run separately.
• A separate workspace build compiled and linked, but its final replacement of the running server executable failed because Windows held the file open. The running service was left untouched.
• Frontend checks were not run because no frontend files changed.

Before review

• Every commit is signed off (git commit -s).
• cargo fmt --all --check passed.
• cargo clippy --locked --workspace --all-targets -- -D warnings passed.
• The complete serial workspace test run passed; the parallel-run failures and live-test scope are disclosed above.