feat(Observability): enable source maps and error stack capture

[jlian]

Now that we're on Workers (#186), we can enable source maps and capture error stacks for readable production debugging.

What

1. Source maps: Add upload_source_maps = true to wrangler.toml and --sourcemap to the build:worker script. Cloudflare auto-translates bundled line numbers to original TypeScript files in Workers Logs.

2. Error stacks: Capture error.stack in two places:

   • Middleware catch-all (_middleware.ts) for unhandled errors
   • Route handler catch blocks via route.fail() properties
   • Store in properties.stack so it's queryable without bloating top-level log fields

Why

Currently errors only log error.message (e.g. "D1 health check failed: SQLITE_ERROR"). With stacks + source maps, you get the full call chain with original file/line numbers, making production debugging much faster.

Scope

• wrangler.toml: add upload_source_maps = true
• package.json: add --sourcemap to build:worker
• _middleware.ts: add stack to error properties in catch-all
• Route handlers: add stack to route.fail() properties in catch blocks
• OBSERVABILITY.md: update template to include stack capture pattern