refactor: move quantity snapshotting to line engine

[turip]

Summary

• move quantity snapshotting from the billing service into the legacy invoicing line engine
• construct and inject one legacy line engine instance into billing and subscription sync
• give the legacy invoice updater a narrow, local QuantitySnapshotter dependency

Motivation

Snapshotting is owned by line engines themselves. Keeping quantity snapshotting with the legacy invoicing line engine makes that ownership explicit and prepares later split-line-group refactors without routing snapshot behavior through the billing service.

Stacked on #4745.

Validation

• nix develop --impure .#ci -c make test-nocache — 6,156 tests passed, 9 skipped
• fast Go lint on all changed package trees — 0 issues

Greptile Summary

This PR moves quantity snapshotting into the legacy billing line engine. The main changes are:

• Adds snapshotting dependencies and methods to the legacy line engine.
• Shares one legacy line engine between billing and subscription sync.
• Gives the invoice updater a narrow quantity snapshotter interface.
• Updates application and test wiring for the new constructors.

Confidence Score: 5/5

This looks safe to merge.

• No blocking issues found in the changed code.
• The moved snapshotting logic preserves the previous dependencies, validation, error mapping, and concurrency behavior.
• Production and test constructors consistently provide the new engine dependency.

Important Files Changed

Filename	Overview
openmeter/billing/lineengine/quantitysnapshot.go	Moves quantity snapshotting from the billing service to the legacy line engine while preserving validation, metering, and error handling.
openmeter/billing/lineengine/engine.go	Adds the feature, streaming, and concurrency dependencies required for engine-owned snapshotting.
openmeter/billing/service/service.go	Accepts and registers a preconstructed legacy line engine instead of constructing one around the billing service.
openmeter/billing/worker/subscriptionsync/service/reconciler/invoiceupdater/invoiceupdate.go	Replaces the broad billing-service snapshot call with a local quantity snapshotter dependency.
app/common/billing.go	Constructs one legacy line engine and injects it into billing and subscription synchronization.

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Context used:

• Context used - CLAUDE.md (source)
• Context used - AGENTS.md (source)

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