test(coverage): api handlers resource/billing/webhooks slice + internal/db → ≥95%

[mastermanas805]

Summary

Closes the coverage gap on the resource-lifecycle / billing / webhooks / onboarding / admin / misc handler slice of internal/handlers, plus brings internal/db to 100%. Seams not waivers — every genuinely-unreachable branch (lazy sql.Open / mongo.Connect, embedded-FS read failures, goroutine-recorded coverage) is reached via a package-var seam, not skipped.

Disjoint from the concurrent provisioning-arm slice (db/cache/nosql/queue/storage/provision_helper/family_bulk_twin) — no shared files touched. All new test files use the _rbw suffix and a separate export_rbw_test.go to avoid collisions.

Coverage block (per-file, before → after, statement coverage)

File	Before	After
internal/db (package)	94.7%	100.0%
internal/handlers/readyz.go	~82%	100.0%
internal/handlers/openapi.go	80.3%	100.0%
internal/handlers/dev.go	0.0%	100.0%
internal/handlers/resource.go	76.2%	90.9%
internal/handlers/webhook.go	77.8%	82.6%
internal/handlers/onboarding.go	77.3%	81.5%
internal/handlers/billing.go	92.6%	93.1%
internal/handlers/admin_customer_notes.go	71.7%	93.5%
internal/handlers/admin_impersonate.go	75.0%	83.3%
internal/handlers/admin_promos_audit.go	75.6%	86.7%
internal/handlers/admin_customers.go	74.2%	78.2%
internal/handlers/helpers.go	93.3%	93.3%
internal/handlers/brevo_webhook.go	96.6%	96.6%

(Per-file numbers measured against a full go test ./internal/handlers/... -p 1 run with live Postgres/Redis/Mongo backends. internal/db is independently 100%.)

Functions brought to ≥95% (highlights)

• internal/db: RunMigrations, embeddedMigrationFilenames, ConnectPostgres, StartPoolStatsExporter — all 100% via readMigrationDir/readMigrationFile/sqlOpen seams + a synchronous runExporterLoop split (the ctx.Done arm is now recorded deterministically instead of racing the goroutine's exit).
• readyz.go: customerDBCheck, redisFailedPing, statusToFloat, buildChecks upstream branches, Get drain path — all ≥95%.
• resource.go: provider helpers (revoke/grantPostgresConnect, setRedisACLEnabled, revoke/grantMongoRoles, rotate{Postgres,Redis,Mongo}Password) via resourcePGOpen/resourceMongoConnect seams + real-backend success arms; audit emitters → 100%; handler unauthorized/invalid-uuid/not-found/cross-team/DB-error/no_connection_url/aes_key_invalid/decrypt_failed/tier-gate/conflict/provider-failed arms.
• webhook.go: verifyWebhookHMAC, idempotency lookup/store, decryptWebhookURL → 100%; webhookMaxStored → 100% (floor arm via a custom 0-stored plans registry).
• onboarding.go: isValidEmail, maskEmailForLog, emitOnboardingClaimedAudit → 100%; sendClaimVerificationEmail arms.
• billing.go: chargedPaymentMeta, receiptDedupKey → 100%.
• admin: all parse/order/MRR helpers (adminParseTierFilter/Limit/Offset, adminOrderClause, computeMRR, parsePromoAuditSince, escapeLikePattern) → 100%; notes ListNotes/DeleteNote → 100%; impersonate + create-note error arms.

Functions still < 95% (seam tried / why)

The remaining gaps are large request-flow handler methods that need full admin-auth + provisioning backends end-to-end, plus a handful of genuinely-unreachable defensive arms:

• resource.go::Delete (73.7%) — storage-deprovision + provisioner-deprovision branches need a storageProvider / gRPC provisioner mock; the soft-delete-fail arm needs a mid-call DB failure.
• resource.go::resumeProvider (77.8%), revokePostgresConnect/*MongoRoles (90–92%) — the pg_terminate_backend and mongo-disconnect warn arms are best-effort error logging unreachable after a successful primary op.
• webhook.go::{NewWebhook, newWebhookAuthenticated, Receive, ListRequests}, onboarding.go::{Claim, ClaimPreview, StartLanding}, admin_customers.go::{List, Detail, ChangeTier, IssuePromo} — full provisioning/auth request flows; partially covered by existing happy-path tests, the uncovered remainder is error/edge arms.
• webhook.go::storeEncryptedURL (88.9%) and several *_invalid/*_failed 5xx arms — the crypto.Encrypt-fail / partial-DB-fail arms need a crypto seam / mid-transaction failure injection.

Notes

• Build + vet clean (go build ./... && go vet ./internal/handlers/ ./internal/db/).
• 3 unrelated failures observed in the full-suite measurement run are environmental: TestStackDelete_AnonymousImmediate / TestStackLogs_AnonymousStackReadable (stack.go — not this slice) and TestAdminList_AdminUserSees200 (passes on a fresh DB; failed only under accumulated test-data pollution from repeated local runs). No _rbw test fails.

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