Build the binary without cgo (closes #545)

make build now builds with CGO_ENABLED=0, and the image builds through
it. The binary's only cgo code was mattn/go-sqlite3, which
gorm.io/driver/sqlite imports and webhooker never opens: every SQLite
file is opened with modernc.org/sqlite and the connection handed to
gorm. Without cgo, mattn's package compiles to a stub nothing calls.

The Dockerfile builds the binary once. The static relink and its
netgo, osusergo and sqlite_omit_load_extension tags are gone, and with
them the Makefile's GO_LDFLAGS and GO_TAGS, which only the relink used.
The test phase keeps cgo for -race. No README or Dockerfile sentence
says the binary needs cgo any more.

Model: opus-5-5

README.md

@@ -1393,10 +1393,10 @@ It uses:
   management
 - **[go-chi](https://github.com/go-chi/chi)** for HTTP routing
 - **[GORM](https://gorm.io)** for database access with
-  **[modernc.org/sqlite](https://pkg.go.dev/modernc.org/sqlite)** as the runtime
-  SQLite driver. Note: `gorm.io/driver/sqlite` transitively depends on
-  `mattn/go-sqlite3`, which requires CGO at build time (see [Docker](#docker)
-  section)
+  **[modernc.org/sqlite](https://pkg.go.dev/modernc.org/sqlite)**, which is pure
+  Go, as the SQLite driver. `gorm.io/driver/sqlite` also imports
+  `mattn/go-sqlite3`, whose stub for builds without cgo is linked into the
+  binary but never used
 - **[slog](https://pkg.go.dev/log/slog)** (stdlib) for structured logging with
   TTY detection (text for dev, JSON for prod)
 - **[gorilla/sessions](https://github.com/gorilla/sessions)** for encrypted
@@ -2135,9 +2135,7 @@ endpoint. That is the JSON key; the error text for a missing one reads
 `webhook_url is required`, which is the message, not the key.
 
 The database uses the
-[modernc.org/sqlite](https://pkg.go.dev/modernc.org/sqlite) driver at runtime,
-though CGO is required at build time due to the transitive `mattn/go-sqlite3`
-dependency from `gorm.io/driver/sqlite`.
+[modernc.org/sqlite](https://pkg.go.dev/modernc.org/sqlite) driver.
 
 #### Schema
 
@@ -3468,16 +3466,12 @@ fixed independently of the compiler's:
    a step's log, and then fails. `make test` builds this stage alone.
 6. **Builder stage** (`golang:1.26.1-bookworm`) — depends on the lint and test
    phases and the `css-check` and `markdown-check` stages passing (it copies a
-   file from each), runs `make build` (which extracts Alpine.js from `3p/`
-   first), and then rebuilds the binary with `CGO_ENABLED=1` and static linking
-   so it runs on musl, with the build tags `netgo`, `osusergo` and
-   `sqlite_omit_load_extension` so it needs none of glibc's shared libraries at
-   run time. Both builds go through `make build`, the relink adding its
-   `-extldflags` via `GO_LDFLAGS` and its tags via `GO_TAGS`, so neither can
-   drop the `-X` that stamps the version. The version is the `VERSION` build arg
-   if one is given, otherwise derived from the `.git` in the context, and the
-   stage fails if a context with `.git` would stamp an empty version, `dev` or
-   `unknown` (see [Version stamping](#version-stamping)).
+   file from each), and runs `make build` (which extracts Alpine.js from `3p/`
+   first). `make build` compiles with cgo off, so the binary is statically
+   linked and runs on the Alpine runtime image. The version is the `VERSION`
+   build arg if one is given, otherwise derived from the `.git` in the context,
+   and the stage fails if a context with `.git` would stamp an empty version,
+   `dev` or `unknown` (see [Version stamping](#version-stamping)).
 7. **Runtime stage** (`alpine:3.21`) — copies the static binary and
    `deploy/docker-entrypoint.sh`, creates the `/var/lib/webhooker` directory for
    all SQLite databases, exposes port 8080, and includes a health check against
@@ -3489,11 +3483,6 @@ The lint and test phases invoke `gofmt`, `golangci-lint` and `go test` directly
 rather than `make fmt-check`, `make lint` and `make test`: those targets build
 docker stages, which would need a docker daemon inside this build.
 
-The lint phase and the Go stages use Debian rather than Alpine because
-`gorm.io/driver/sqlite` pulls in `mattn/go-sqlite3`, which needs CGO and does
-not compile against musl. Only the final binary is statically linked, which is
-what lets it run on the Alpine runtime image.
-
 `script/cibuild` is the CI gate: it runs `script/bootstrap`, then
 `script/check`, then builds the image, whose build runs the lint and test phases
 and the stylesheet and Markdown checks again. A build that succeeds is a repo