chore: prepare release v2.8.0

docs/releases/v2.8.0.md

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+# MTG Python Deckbuilder v2.8.0
+
+### Summary
+Theme catalog improvements with faster processing, tag search features, regeneration fixes, and browser performance optimizations.
+
+### Added
+- **Theme Catalog Optimization**:
+  - Consolidated theme enrichment pipeline (single pass instead of 7 separate scripts)
+  - Tag index for fast theme-based card queries
+  - Tag search API with new endpoints for card search, autocomplete, and popular tags
+  - Commander browser theme autocomplete with keyboard navigation
+  - Tag loading infrastructure for batch operations
+- **Theme Browser Keyboard Navigation**: Arrow keys now navigate search results (ArrowUp/Down, Enter to select, Escape to close)
+
+### Changed
+- **Theme Browser Performance**: Theme detail pages now load much faster
+  - Disabled YAML file scanning in production (use `THEME_CATALOG_CHECK_YAML_CHANGES=1` during theme authoring)
+  - Cache invalidation now checks theme_list.json instead of scanning all files
+- **Theme Browser UI**: Removed color filter from theme catalog
+
+### Fixed
+- **Theme Regeneration**: Theme catalog can now be fully rebuilt from scratch without placeholder data
+  - Fixed "Anchor" placeholder issue when regenerating catalog
+  - Examples now generated from actual card data
+  - Theme export preserves all metadata fields