[DOP-27960] Move to dataset symlink group

[IlyasDevelopment]

Change Summary

Introduce fingerprint-based dataset symlink group storage while preserving the old directed symlink interface.

• Add dataset_symlink_group table and migrate existing dataset_symlink rows into fingerprinted group members.
• Replace the physical dataset_symlink table with a compatibility view that reconstructs the old from_dataset_id/to_dataset_id/type shape.
• Add DatasetSymlinkGroupDTO and deterministic symlink fingerprint computation.
• Update consumer extraction flow to emit symlink groups instead of directed symlink pairs.
• Update BatchExtractionResult and DatabaseSaver to deduplicate and bulk-insert symlink groups.
• Keep server lineage read behavior one-hop via the dataset_symlink compatibility view.
• Update seed data, factories, and tests for group-based symlink writes.
• Add migration coverage for backfill correctness, fingerprint grouping, role assignment, and compatibility view output.

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[github-actions]

Coverage Report •

File	Stmts	Miss	Branch	BrPart	Cover	Missing
data_rentgen/consumer
saver.py	160	13	46	4	91%	42–43, 183–189, 191–192, 195–196
data_rentgen/consumer/extractors/impl
flink.py	37	1	6	1	97%	42->43, 43
spark.py	64	1	14	2	98%	88->90, 108->109, 109
data_rentgen/db/migrations/versions
2026-06-29_4a02d2d5c8b1_create_dataset_symlink_group.py	37	37	8	8	0%	11–12, 14, 21, 24–27, 30–31, 48–50, 111–114, 117–119, 121, 145, 153–155, 158–159, 161–163, 165, 173, 182, 186, 193, 201–202
data_rentgen/db/models
dataset_symlink.py	17	1	0	0	94%	21
TOTAL	8225	1070	1320	238	86%