* docs: apply mechanical doc-review fixes (#565)

* docs: fix 30 broken links, 11 factual errors, 5 ssot redundancies

Applies fixes from DOC_REVIEW_REPORT.md (2026-04-14 audit, 80 files).

Phase 1 (broken links & anchors):
- Repaired README paths to split FEATURES_ORM_QUERY.md (query-builders,
  orm-framework anchors) and current ARCHITECTURE/API_REFERENCE locations.
- Fixed language-switcher links pointing at non-existent .kr.md files
  (ADAPTER_PATTERNS, ASYNC_OPERATIONS, INTEGRATION, UNIFIED_SYSTEM).
- Repaired relative paths in docs/advanced/* and docs/guides/* that
  produced docs/advanced/docs/... style dead links.
- Marked missing targets (proxy-mode.md, IMPROVEMENT_PLAN.md,
  backend_registry.md, POSTGRESQL_TUNING.md, SQLITE_TUNING.md,
  advanced/SECURITY.md, PROXY_LAYER.md, with-*-system.md) as TODO.

Phase 2 (factual accuracy & Phase 4.3 pool-removal propagation):
- Corrected backend list to PostgreSQL/SQLite/MongoDB/Redis
  (MySQL never existed) in CURRENT_STATE and ARCHITECTURE_ISSUES.
- Aligned compiler baseline to GCC 13+/Clang 17+/MSVC 2022+/Apple
  Clang 14+ across README, CONTRIBUTING, GETTING_STARTED, samples.
- Updated libpqxx 7.9.0 -> 7.9.2 and OpenSSL 3.3.0 -> 3.4.1 in SOUP
  to match vcpkg.json overrides; refreshed PROJECT_STRUCTURE versions.
- Changed ORM description from C++17 SFINAE to C++20 concepts across
  ORM_GUIDE and FEATURES_ORM_QUERY.
- Propagated Phase 4.3 pool removal to English docs: deprecation
  banners in FEATURES_POOLING_SECURITY and BACKENDS; ADR-002 marked
  Superseded.

Phase 3 (SSOT & registry):
- Regenerated docs/README.md registry: was 59 docs, now enumerates
  66 docs under docs/ plus the 14 out-of-tree repo Markdown files
  (total 80), adding FEATURES_BACKENDS, FEATURES_ORM_QUERY,
  FEATURES_POOLING_SECURITY, API_QUICK_REFERENCE, ECOSYSTEM,
  GETTING_STARTED, and the registry itself.
- Resolved 3 SSOT pairs by deferral: advanced/ARCHITECTURE and
  advanced/STRUCTURE defer to docs/ root; performance/BENCHMARKS
  defers to docs/BENCHMARKS. Added bidirectional See also xrefs.
- Added xrefs ORM_GUIDE <-> TYPE_SYSTEM/API_QUICK_REFERENCE and
  FEATURES_BACKENDS -> ADAPTER_PATTERNS.

Skipped by policy: Korean (.kr.md) sync, API/manifest version bumps,
BENCHMARKS en/kr language-pair duplication, docs/README.kr.md path
corrections (Korean file).

* docs: add post-fix re-validation report

* docs: fix pool-removal banner anchor regression

* feat(release): prepare database_system for v1.0.0 (#566)

* refactor(api): replace throw with Result<T> in unified_database_system

Align implementation with documented no-throw guarantee:
- Defer coordinator initialization to connect() via lazy init pattern
- Change builder::build() return type to Result<unique_ptr<>>
- Add ensure_initialized() private method for deferred init

Closes #564

* refactor(tests): update callers for Result<T> builder API

Update all test and sample code to handle
Result<unique_ptr<unified_database_system>> return type from
builder::build() instead of raw unique_ptr.

* chore: bump version to v1.0.0 and update documentation

- Bump version in CMakeLists.txt, database/CMakeLists.txt, vcpkg.json
- Update CHANGELOG with v1.0.0 release notes and comparison links
- Add v1.0.0 migration notes to README
- Fix pre-existing broken markdown anchors in docs

* fix(api): correct compile errors in builder Result return type

- Fix error_info field name: source -> context
- Update create_database() convenience function return type to match
  builder::build() Result<unique_ptr<>> signature

* fix(api): use make_error_result helper for correct error_info construction

The common_system error_info struct does not have a 'context' member
accessible via .context. Use the existing make_error_result helper
which constructs error_info correctly with positional arguments.

* fix(samples): move db variable to outer scope in migration sample

The db variable was declared inside an if block, making it
inaccessible to subsequent code that uses db->execute(),
db->get_metrics(), and db->check_health().