Add idempotent PostHog import

[nicosuave]

Adds a host-side PostHog importer for persons, groups, and historical events.

• preserves PostHog event UUIDs and uses deterministic UUIDs for synthetic person/group import rows
• requires R2 SQL target checks by default for real imports, with explicit opt-out for local testing
• filters each batch against local state and the target table before sending
• handles ambiguous pipeline sends by polling R2 SQL using a flush-aligned wait instead of blindly retrying
• includes generic helper scripts for missing UUID checks and imported-row duplicate cleanup, configured by args/env rather than live identifiers

Validation:

• cargo fmt --all --check
• cargo test --lib
• uv run python -m py_compile scripts/dedupe_imported_posthog_rows.py scripts/find_missing_posthog_uuids.py
• cargo test still fails at tests/persons_do.rs because build/index.js is missing and that test asks to run worker-build first

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