test: add multi-tenant isolation property tests across plans, subscriptions, and statements (#782)

[supreme2580]

Implements property-style integration tests for multi-tenant isolation across plans, subscriptions, and statements as requested in #782.

Changes

Test Implementation

• New test file: internal/service/tenant_isolation_property_test.go with property-based isolation tests
• Two-tenant fixture seeding: Tenants tenant-alpha and tenant-beta with overlapping subscription IDs (sub-a-, sub-b-) and statement IDs (stmt-a-, stmt-b-)
• 200+ randomized requests per endpoint with fixed seed (42) for deterministic failure reporting

Test Coverage

1. SubscriptionService.GetDetail cross-tenant isolation
2. StatementService.ListByCustomer cross-tenant isolation
3. PlanRepository.List tenant scoping

Code Quality Fixes

• Fixed syntax errors in featureflags, partition, statements, audit, svid, otel_handler
• Fixed import issues: gin-gonic/gin typo, brotli import path (klauspost->andybalholm)
• Updated go.mod/go.sum with andybalholm/brotli v1.2.3

Test Properties Verified

• Deterministic: Fixed seed (42) for reproducible failures
• Property-based: 200+ randomized requests per endpoint
• Isolation guarantees: Cross-tenant access always returns ErrNotFound/nil
• Overlapping IDs: Tenant A and B have colliding ID patterns to stress-test isolation

Closes #782

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