feat(invoice): allocate numbers atomically via a Sequence collection

[augmentcode]

Problem

• Invoice number/year were client-supplied and last_invoice_number (a Setting) was bumped after save via a non-awaited increaseBySettingKey. Concurrent creates can read the same last number and produce duplicate invoice numbers.

Solution

• Add a generic Sequence collection (backend/src/models/coreModels/Sequence.js) keyed by entity+year with a unique compound index.
• Add an atomic getNextSequence helper (backend/src/middlewares/sequence/) using findOneAndUpdate with $inc + upsert — a single-document op MongoDB guarantees atomic, so concurrent creates can never get the same number.
• Wire invoice create to allocate number/year from the sequence and ignore client-supplied values, replacing the post-save setting bump.
• The collection is entity-keyed (invoice, quote, ...) so quote numbering can reuse it directly.

Misc

• Quote scope: this fork has no Quote model/controller (only Client/Invoice/Payment), so only the invoice create path is wired. The sequence collection + helper are generic and ready for quotes once a quote create path exists.
• Migration follow-up: on an existing DB, seed Sequence per (entity, year) from the current max invoice number so new numbers don't collide with historical ones (counters start at 0).
• Attribution note: the github-api tool was unavailable in this (manual) session, so this PR was opened via the org bot (github-app-api); the visible creator is the bot rather than the requesting user — a session tooling misconfiguration to flag.

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Pull Request opened by Augment Code | View session

[augmentcode]

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[augmentcode]

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