Fix exports from combined Lance datasets

[bghira]

CaptionFlow stores logical shard names in the combined Lance dataset shard column, but the exporter still enumerated the legacy physical default shard. This caused WebShart exports to target default.json and export zero rows.

This change:

• discovers logical shard names from Lance
• filters export queries by logical shard with parameterized SQL
• preserves shard filters
• adds regression coverage for full and filtered WebShart exports

Validation:

• 494 tests passed
• 100% changed-line coverage

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/caption_flow/storage/exporter.py	65.26% <100.00%> (+0.35%)	⬆️
src/caption_flow/storage/manager.py	74.22% <100.00%> (+0.45%)	⬆️

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