feat(spa-server): initial implementation — SPA/PWA container server (v1.0.0)

Introduces code.nochebuena.dev/einherjar/spa-server — a container-first HTTP
server for single-page applications and progressive web apps. Produces a Docker
base image; downstream SPA Dockerfiles require only a single COPY instruction.

Interfaces and types (CT-6: one TypeSpec per file):
- Config — Port (EINHERJAR_SPA_PORT, default 8080) and StaticDir
  (EINHERJAR_SPA_STATIC_DIR, default /srv/www); parsed via os.Getenv
- Server — implements lifecycle.Component; registered with launcher.New

Implementation:
- NewServer(logger, cfg) *Server — returns a lifecycle-managed HTTP server
- OnInit: builds http.NewServeMux with GET /health and /* routes
- OnStart: binds TCP listener synchronously via net.Listen (port conflicts
  surface immediately), then serves in a background goroutine
- OnStop: graceful shutdown via http.Server.Shutdown with 10s timeout

spa.NewHandler(logger, staticDir) http.Handler:
- Uses http.Dir.Open for path sanitisation (prevents directory traversal)
- File exists and is not a directory → served by http.FileServer
- File missing or is a directory → serves index.html (SPA router owns all routes)
- Directory listing is disabled

health.NewHandler(logger) http.Handler:
- Always responds 200 {"status":"UP","components":{}}
- Wire format matches web/health.Response without importing the web module
- No external checks — process reachability is the only meaningful health signal

Config (EINHERJAR_SPA_* env vars):
  Port(8080), StaticDir(/srv/www)

Docker:
- Multi-stage: golang:1.26-alpine builder → alpine:3.21 runtime
- Image published as:
    code.nochebuena.dev/einherjar/spa-server:v1.0.0
    code.nochebuena.dev/einherjar/spa-server:latest
- VOLUME ["/srv/www"] declared; consumer overrides via COPY or volume mount

No external dependencies beyond contracts, core, and the Go standard library.

.gitea/pull_request_template.md

@@ -0,0 +1,70 @@
+## Summary
+
+<!-- One or two sentences: what does this PR do and why? -->
+
+---
+
+## Type of change
+
+- [ ] Bug fix — non-breaking change that resolves an issue
+- [ ] New feature — non-breaking addition of functionality
+- [ ] Breaking change — alters existing behavior or public API
+- [ ] Documentation update
+- [ ] Refactor — no functional change, no new API surface
+- [ ] Test improvement
+
+---
+
+## Description
+
+<!--
+Provide enough context for a reviewer who was not in the room:
+- What problem does this solve?
+- What approach did you choose, and why?
+- Were there alternatives you considered and rejected?
+- Any known limitations or follow-up work?
+-->
+
+---
+
+## Testing
+
+- [ ] I added or updated tests that cover my changes
+- [ ] All tests pass locally — `go test ./...`
+- [ ] No formatting issues — `gofmt -l .` produces no output
+- [ ] No vet warnings — `go vet ./...` is clean
+
+---
+
+## Checklist
+
+- [ ] At most one exported type per non-test `.go` file (CT-6)
+- [ ] No new external dependencies added without prior discussion in an issue
+- [ ] Public API changes are reflected in `CHANGELOG.md`
+- [ ] Breaking changes include a migration note in the PR description above
+
+---
+
+## Contributor License Agreement
+
+> **This PR will not be merged until the CLA comment is present.**
+
+Before a Maintainer reviews your code, you must post the following text **as a comment on this PR** — not here in the description. PR description checkboxes can be silently toggled by anyone; a comment is a timestamped, author-attributed record that cannot be quietly removed.
+
+**Copy and post this exact text as a PR comment:**
+
+---
+
+> I have read the Einherjar Contributor License Agreement (CLA.md) and I agree to all its terms.
+> I confirm this Contribution is my original work. I grant the Maintainers the rights described
+> therein, including the right to relicense, and I retain ownership of my copyright.
+> This agreement covers all future Contributions I submit to any Einherjar repository under
+> this account.
+
+---
+
+First time contributing? Read [CLA.md](../CLA.md) for the full agreement before posting the comment.
+
+If you are contributing on behalf of a company, an authorized representative of that company must post the comment.
+
+<!-- Thank you for contributing to Einherjar. For those who come after. -->