refactor: restructure backend src as library with clear module boundaries
Move server.zig, search.zig, dashboard.zig into server/ directory.
Move reconcile.zig into ingest/ as reconciler.zig alongside other
background workers. Replace top-level ingest.zig and metrics.zig
shims with mod.zig inside their directories. Root tests at main.zig
with shared imports to fix zig 0.15 module path boundary.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>