feat(api): add internal storage limit grant for billing

[MehrshadFb]

Summary

• Add PhotoStorageService.addStorageLimit(userId, additionalBytes): one Prisma increment on User.storageLimitBytes, returning the new limit as a bigint
• Reject a non-positive or fractional increment with 400 before any database work, and map Prisma's P2025 to 404 for an unknown user
• Emit user.storage_limit.increased with audit: true and both byte counts as strings, since bigints are not JSON-serializable
• Extract the cause-chain walk already used for serialization failures into a shared someCause() helper, now backing both isSerializationFailure and the new isRecordNotFound
• Document the method in photos-architecture.md §9 in place of the raw Prisma snippet, and split the billing checklist into the part that now exists and the part that does not

Why

The per-user ceiling landed in #42, but raising one was still a hand-written UPDATE documented as a snippet. This turns that snippet into the single call a future payment webhook makes, so the billing layer never has to know the column exists.

Additive rather than absolute is the load-bearing choice. A purchase grants capacity instead of declaring a total, so two overlapping grants accumulate, and increment compiles to one statement, leaving no read-modify-write window for a grant to be lost in.

Notes

• Stacked on feat/per-user-storage-limit (feat(api): store photo storage limit per user #42), which must merge first; retarget to main once it does
• Internal only, exactly as scoped. No controller route, no Stripe or billing module, no downgrade or set-absolute path. A downgrade needs its own rules for usage already above the new ceiling, so it is deliberately not bundled here
• Idempotency stays with the caller. A webhook redelivered twice grants twice, and deduping by payment event id is the payment layer's job
• No OpenAPI or mobile change, since nothing new is reachable over HTTP

Test plan

• npx jest --watchman=false (279 tests, 16 suites)
• npx jest --config ./test/jest-e2e.json --watchman=false (98 tests)
• npm run typecheck
• npm run openapi:check
• npx eslint on changed files, npm run format:check
• CI passes on PR

[github-actions]

API unit-test coverage

Metric	Coverage
Statements	68.14% (629/923)
Branches	50.09% (261/521)
Functions	72.67% (117/161)
Lines	66.46% (549/826)

Unit suite only; controllers are exercised by the e2e suite.