perf: Reduce database, build, and homepage overhead

[Fuwn]

Reduce storage overhead in new SQLite databases, improve Docker build caching, and avoid unnecessary homepage downloads. Prepare 0.1.29 while preserving existing counter behaviour.

Changes

• Remove the redundant identity index from new SQLite tables while retaining identity uniqueness, AUTOINCREMENT, and unique counter names.
• Reuse the cached theme layer for source-only Docker builds. Changes to themes or their synchronisation script still invalidate it.
• Drop unused font styles, use textContent for embed snippets, and defer mascot loading until its viewport breakpoint becomes visible. Preserve snippet escaping, newlines, resize handling, and older media-query listeners.
• Bump the package version to 0.1.29.

Compatibility

This change introduces no database migration. Existing SQLite databases retain their data, schema, and indexes; database storage savings apply to newly created tables. Counter increment SQL, PostgreSQL behaviour, response caching, name encoding, and SVG rendering remain unchanged.

Validation

• Formatting, compilation, all 10 Gleam regression tests, PostgreSQL 17 integration checks, and Nix builds on Linux and macOS passed.
• ARM64 and AMD64 container builds and HTTP checks passed. AMD64 execution used emulation. Hosted CI passed all four jobs.
• Existing SQLite data, timestamps, schema, and indexes were preserved in compatibility checks using disposable database copies. Concurrent increments, restart, and rollback to 0.1.28 passed.
• Mobile and desktop browser checks passed for snippets, escaping, resizing, and older media-query listeners. External image-host failures limited mascot rendering checks; Safari was not tested.
• Measured workloads showed no reproducible server CPU, memory, or throughput regression. New-database size decreased by 14%, and decoded font stylesheet data decreased by 84%.