feat(storage): add built-in storage engine with system-level fallback

[nixidexiangjiao]

Description

Add a declarative system-level fallback mechanism for storage engine configuration via config/builtin_storage_engine.yaml, mirroring the existing builtin_models.yaml pattern. When a tenant's storage_engine_config is absent or has an empty provider sub-config, the system automatically falls back to the built-in config.

Key changes:

• Unified resolver layer (internal/types/storage_resolve.go): ResolveXxxConfig functions implement tenant → builtin → env fallback with empty-shell detection. If a tenant sub-config has all key fields empty (e.g. OSS: {}), it's treated as an empty shell and bypassed in favor of the builtin fallback.
• YAML loader (internal/types/builtin_storage_engine_config.go): Loads the config file at startup with ${ENV} and ${ENV:-default} placeholder interpolation, stored in an atomic.Pointer singleton.
• Env interpolation (internal/types/env_interpolation.go): ${VAR} for string fields (unset stays literal to surface misconfig), ${VAR:-default} for bool/int fields (unset falls back to default to avoid YAML parse errors).
• Factory & handler refactor: Replace scattered sec == nil || sec.Xxx == nil checks with types.ResolveXxxConfig(sec) calls in factory.go and system.go.
• Frontend fix: KBStorageSettings.vue now checks props.storageProvider instead of local state for provider pre-selection.
• OBS deprecation: The legacy OBS_* env-var path is now the 3rd-tier fallback (tenant → builtin → env) and emits a sync.Once deprecation warning. Planned for removal in vip-v0.7.x.
• YAML struct tags: All *EngineConfig types now have yaml tags alongside existing json tags, enabling deserialization from both API/DB (JSON) and config file (YAML).

Type of Change

• 🐛 Bug fix
• ✨ New feature
• 💥 Breaking change
• 📚 Documentation update
• 🎨 Refactor
• ⚡ Performance improvement
• 🧪 Test
• 🔧 Configuration / Build / CI

Related Issue

Fixes #

Testing

All new test files pass with go test ./internal/types/ ./internal/application/service/file/ ./internal/handler/:

• internal/types/env_interpolation_test.go — ${VAR} and ${VAR:-default} expansion, unset/empty/default cases
• internal/types/storage_resolve_test.go — Full matrix: tenant-first, empty-shell fallback, partial-config-no-fallback, both-nil; plus OBS env deprecation and sync.Once once-only guarantee
• internal/types/builtin_storage_engine_config_test.go — File missing, invalid YAML, env interpolation, empty config, env override path, atomic replace
• internal/application/service/file/factory_builtin_test.go — All 8 providers with builtin fallback, default provider fallback, both-nil returns incomplete
• internal/handler/system_storage_resolve_test.go — isXxxConfigured with builtin fallback for all providers
• internal/handler/tenant_storage_resolve_test.go — GetTenantStorageEngineConfig fills DefaultProvider from builtin, respects STORAGE_ALLOW_LIST, never leaks builtin secrets to client

Checklist

• make fmt && make lint && make test pass locally
• Self-reviewed the code
• Added/updated tests covering the change
• Updated related documentation (README, docs/, Swagger annotations, etc.)
• Breaking changes are clearly called out in the description above

Screenshots / Recordings

No user-visible UI changes (only a minor fix to provider pre-selection logic in the KB storage settings).