feat: read gathering lines from dedicated table

[turip]

Summary

• make gathering-line DB state record whether a line came from billing_invoice_lines or billing_gathering_invoice_lines
• read and merge gathering lines from both tables while rejecting duplicate IDs
• keep standard-line and gathering-line subscription reads behind their respective adapter and service contracts
• update subscription sync to load both standard persisted state and gathering leaves
• expand split-line hierarchy mapping across both line tables
• keep dedicated-table writes disabled during this read-compatible migration stage

Stack

Depends on #4720, which introduces the dedicated gathering invoice line schema.

Impact

This is the first migration iteration: existing gathering lines remain writable through the legacy table, while lines already migrated to the dedicated table become readable by billing and subscription sync. Conflicting rows in both tables fail explicitly instead of being resolved implicitly.

Validation

• make test-nocache

Greptile Summary

This PR adds read compatibility for gathering lines stored in either invoice-line table. The main changes are:

• Records the source table in each gathering line snapshot.
• Reads and merges legacy and dedicated gathering lines.
• Rejects duplicate line IDs across the two tables.
• Splits standard and gathering subscription reads.
• Loads split-line hierarchies from both tables.
• Keeps dedicated-table writes disabled during migration.

Confidence Score: 5/5

This looks safe to merge.

• Split-group members are excluded from standalone gathering reads and remain available through their hierarchy.
• Both subscription reads retain the transaction-bearing lock context.
• No blocking issue remains in the updated code.

Important Files Changed

Filename	Overview
openmeter/billing/adapter/gatheringlines.go	Adds dual-table gathering-line reads, source-aware snapshots, duplicate checks, and split-group filtering.
openmeter/billing/adapter/invoicelinesplitgroup.go	Extends split-line hierarchy mapping to dedicated gathering lines.
openmeter/billing/adapter/stdinvoicelines.go	Separates standard subscription reads while retaining split-line hierarchy loading.
openmeter/billing/worker/subscriptionsync/service/persistedstate/loader.go	Combines standard persisted state with standalone gathering leaves.

_{Reviews (3): Last reviewed commit: "fix: exclude split gathering lines from ..." | Re-trigger Greptile}

Context used (3)

• Context used - CLAUDE.md (source)
• Context used - AGENTS.md (source)
• Context used - api/spec/AGENTS.md (source)

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✨ Finishing Touches

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• Commit unit tests in branch feat/migrate-gathering-invoice-lines

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