bring repo into policy compliance; vendor assets; Gitea CI

Vendor the front-end assets and drop the third-party CDN dependencies
(BootstrapCDN is being sunset): the bootstrap 4.0.0 css/js, jquery
3.2.1 slim, and popper 1.12.9 now live under static/ and are served
from the app, byte-for-byte identical to the previous SRI-pinned files.

Migrate CI from Drone to a Gitea Actions workflow that runs
docker build . on push, with the checkout action pinned by SHA.

Bring the repo up to standard:

- add REPO_POLICIES.md, .editorconfig, .dockerignore, .golangci.yml,
  and a comprehensive root-anchored .gitignore
- rewrite the Makefile with the required test/lint/fmt/fmt-check/check/
  docker/hooks targets (golangci-lint, 30s test timeout, verbose rerun
  on failure, check modifies nothing)
- rewrite the Dockerfile as a hash-pinned multistage build: a lint
  stage (golangci-lint), a glibc build+test stage (the legacy sqlite
  driver needs cgo+glibc), and a debian-slim runtime carrying the
  binary, templates, and static assets
- add real tests for the hn package
- bring the code into golangci-lint (default: all) compliance: fix the
  malformed gorm struct tags, check previously-ignored errors, dispatch
  the zerolog error event, avoid a uint->Duration overflow, split long
  functions, and add doc comments — all behaviour-preserving
- expand the README with the required sections

cmd/server/main.go

@@ -1,4 +1,4 @@
-//go:generate go run github.com/UnnoTed/fileb0x b0x.yaml
+// Command server runs the orangesite transparency-log web application.
 package main
 
 import (
@@ -7,8 +7,14 @@ import (
 	"git.eeqj.de/sneak/orangesite/hn"
 )
 
-var Version string
-var Buildarch string
+// Version and Buildarch are injected at build time via
+// -ldflags "-X main.Version=... -X main.Buildarch=...".
+//
+//nolint:gochecknoglobals // set at link time by the build, cannot be const
+var (
+	Version   string
+	Buildarch string
+)
 
 func main() {
 	os.Exit(hn.RunServer(Version, Buildarch))