docs: replace nginx with Caddy across all documentation and installer

CONTRIBUTING.md

@@ -18,7 +18,7 @@ See [Cutting a release](#cutting-a-release) for the full procedure.
 
 - **Go 1.26+** (uses Go 1.22+ `http.ServeMux` method patterns and `log/slog`)
 - [`just`](https://github.com/casey/just) command runner (the `justfile` is the source of truth for build / test / run / install / uninstall)
-- Linux, macOS, or FreeBSD
+- Linux or FreeBSD
 - A local SMTP account for end-to-end testing. Tests in `internal/storage` and `pkg/contactform` do not require SMTP.
 
 ### Quick Start
@@ -42,8 +42,15 @@ just test
 
 The suite covers:
 
-- `pkg/contactform` — validators for all four form types (contact, feedback, newsletter, generic), whitespace trimming, and error message shape.
+- `cmd/server` — client IP extraction, request-logging middleware.
+- `internal/config` — TOML loading, env-var expansion, schema validation.
+- `internal/email` — SMTP message composition (`multipart/alternative`) and subject templates.
+- `internal/handler` — form handler pipeline (CORS, rate limit, honeypot, validation, send), rate-limiter cleanup loop.
 - `internal/storage` — `Append` / `Count` / `List` semantics, file and directory auto-creation, missing-file tolerance, malformed-line resilience.
+- `internal/version` — version identity and `--version` smoke output.
+- `pkg/contactform` — validators for all four form types (contact, feedback, newsletter, generic), whitespace trimming, and error message shape.
+
+CI enforces a **≥80 % coverage threshold** on `internal/...` and `pkg/...`. Run `go test -coverprofile=coverage.out ./internal/... ./pkg/... && go tool cover -func=coverage.out` locally to check before pushing.
 
 ### Linting
 
@@ -151,7 +158,7 @@ nuntius/
 ```mermaid
 graph TD
     Browser[Browser / Frontend]:::accent6
-    Nginx[nginx reverse proxy]:::accent6
+    Caddy[Caddy reverse proxy]:::accent6
     CLI[cmd/server/main.go]:::accent0
     Config[internal/config/]:::accent7
     Handler[internal/handler/]:::accent0
@@ -161,8 +168,8 @@ graph TD
     SMTP[(SMTP server)]:::accent2
     JSONL[(data_dir/newsletter-*.jsonl)]:::accent2
 
-    Browser -->|POST /api/nuntius/contact| Nginx
-    Nginx -->|forward with X-Forwarded-For| CLI
+    Browser -->|POST /api/nuntius/contact| Caddy
+    Caddy -->|forward with X-Forwarded-For| CLI
     CLI --> Config
     CLI --> Handler
     Handler --> Validate
@@ -170,8 +177,8 @@ graph TD
     Handler --> Storage
     Email -->|SMTP PLAIN| SMTP
     Storage -->|append JSONL| JSONL
-    Handler -->|200 ok / 4xx / 5xx| Nginx
-    Nginx --> Browser
+    Handler -->|200 ok / 4xx / 5xx| Caddy
+    Caddy --> Browser
 
     classDef accent0 fill:#3B82F6,stroke:#2563EB,color:#fff
     classDef accent1 fill:#22C55E,stroke:#16A34A,color:#fff
@@ -212,7 +219,7 @@ Triggered on every push to `development` and on every pull request targeting `de
 | Job | What it does | Why it exists |
 |---|---|---|
 | `vet` | `gofmt -l cmd internal pkg` (must print nothing) and `go vet ./...` | Catches formatting drift and static-analysis issues before tests run. |
-| `test` | `go test -race -count=1 ./...` plus `go build ./examples/...` | Race detector catches issues in the per-form rate limiter, the storage mutex, and the SMTP send path. `-count=1` disables the result cache. The example build is a smoke test for the public library. |
+| `test` | `go test -race -count=1 ./...`, coverage gate (`≥80 %` on `internal/...` and `pkg/...`), plus `go build ./examples/...` | Race detector catches issues in the per-form rate limiter, the storage mutex, and the SMTP send path. `-count=1` disables the result cache. The coverage gate fails the build if the combined statement coverage drops below 80 %. The example build is a smoke test for the public library. |
 | `build` | `just build`, `just test`, and `./bin/nuntius --version` smoke test | Confirms the `justfile` build recipe works end-to-end and that `--version` still prints the expected output. |
 
 Run the same checks locally before opening a pull request: