Dedupe custom field definitions on re-import

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Summary

Restoring a Sheaf export into a system that already had its custom fields stacked a second copy of every definition ("Pronouns", "Pronouns", ...), because the importer created definitions unconditionally. This dedupes them. Follow-up to the import-completeness work merged in #74; sequenced as unreleased (not folded into v0.2.2).

Change

• Custom field definitions are deduped by (name, type) against the target system and within the file itself, reusing the existing definition rather than minting a twin. The system's own config (privacy / order / options) on a reused field is left untouched.
• Members are still added rather than merged (member dedup is separate, larger work), so their values attach to the fresh member rows under the shared definition.
• Field-value inserts guard (field_id, member_id) uniqueness in-run, so a reused definition (a file with duplicate defs, or future deduped members) can't trip the UNIQUE(field_id, member_id) constraint mid-import.

Also

• Syncs uv.lock to 0.2.2 (the v0.2.2 bump updated pyproject but not the lockfile; only the version field moves, no dependency resolutions change). Same lockfile-drift class as the package-lock.json fix in v0.2.2.

Tests

• New runner test: importing the same export twice creates the field once, the second run reports zero new fields, and exactly one definition survives.
• 11 passed across the import runner + preview suite; ruff clean.

Not in scope

Member deduplication (re-import stays additive for members).