[feat](web): serve the SPA and web actions on the single listener

Add the browser edge (contract rev 8.2/9): a Preact/Vite SPA under web/,
an embedded placeholder bundle, the /web/* auth and action handlers, and
the explicit browser-action allowlist. The SPA and the API share the one
plaintext :8080 listener; there is no second port and no peer edge, so
main.go also drops the mTLS listener and the identity mounts in the debug
compose fall away. /_manifest announces the web block only while the SPA is
mounted (honesty rule), always with the single service port.

The DSNs move to the shared database's scram-sha-256 credentials, matching
rev 9's removal of the PKI layer.

Committed web/bun.lock because web/package.json and vite.config.ts are new
in this commit. src/web/dist/index.html is a hand-written placeholder, not
a generated bundle: it keeps bare `go build`/`go vet` working from a clean
checkout, while /web/dist (the real build output) stays gitignored.

.gitignore

@@ -2,3 +2,19 @@
 src/tmp/
 src/src/
 src/logs/
+
+# Frontend build output and dependencies (the committed placeholder at
+# src/web/dist/index.html is deliberately NOT ignored — it keeps bare
+# `go build`/`go vet` working from a clean checkout). Patterns are anchored to
+# the repo root so `src/web/dist/` stays tracked.
+/web/dist/
+/web/node_modules/
+/web/.bun/
+/web/.buntmp/
+
+# Go build cache (sandbox: default GOCACHE is read-only)
+/.gocache/
+/.gotmp/
+/.docker-config/
+/.vision/
+/.vision/