tooling: adopt go tool directives; rename air→dev; bundle fmt drift

- Justfile: rename `air:` to `dev:`, body uses `go tool air`; the three
  migrate recipes use `go tool migrate`; `fmt` adds `go fix ./...`.
  Stale `brew install golang-migrate` comment block replaced with the
  one-line `go get -tool` bootstrap hint.
- README.md: quickstart says `just dev`.
- go.mod: declare github.com/air-verse/air and golang-migrate's cmd
  in the `tool` block (Go 1.24+) so contributors never need a separate
  install step. Updates go.sum accordingly.
- internal/domain/stats/repository.go: pre-existing `HostSplit` indent
  drift + trailing newline, surfaced by goimports inside the new fmt
  recipe.

docker: stabilize web-builder paths and bump builder Go to 1.26

Two related Dockerfile correctness fixes uncovered while validating the
review pass:

1. The web-builder stage used WORKDIR /web with vite's relative
   outDir ../internal/server/web/dist resolving to /internal/server/web/dist
   at the container root — working only by relative-path coincidence.
   Mirror the host repo layout inside /src so the path is /src/internal/server/web/dist
   in both stages, making the COPY --from explicit and stable against any
   future WORKDIR or outDir change.

2. golang:1.25-alpine no longer compiles the codebase: the auxilia/culpa
   dep uses errors.AsType which is in 1.26+. Bump the builder image to
   golang:1.26-alpine and the go.mod directive from 1.25.0 to 1.26.0
   so they agree on the actual minimum.

Verified: docker build (full multi-stage) green; produced binary runs.

feat(config): viper-loaded config with fail-fast validation

feat: bootstrap lethe server skeleton + wire contract