feat: add web layer (internal/web) and wire it into main.go

Routing, html/template layout/content pattern, and the Pre-Publish
check flow: POST /pruefen runs extraction (Stufe 1) then rules.Evaluate
(Stufe 2) and renders the result as an htmx fragment. Nothing is
persisted yet — that's the next step (wiring internal/store in).

The needsClarification case is rendered explicitly as a request for
more information rather than "no findings", matching the core
principle. Every result carries the legal-advice disclaimer required by
CLAUDE.md's guardrails.

Server depends on a narrow Extractor interface rather than *extract.
Client directly, so tests inject a fake instead of calling the real API
— internal/web's test suite never touches the network. htmx is vendored
locally (internal/web/static/htmx.min.js) instead of loaded from a CDN,
keeping the UI usable without runtime internet access.

cmd/deklarix/main.go now wires all of this together: reads
ANTHROPIC_API_KEY (required) and RULES_DIR (default "rules"), builds
the extract client and loads the rule set, and serves web.Server instead
of the old inline health-only mux.

This exposed the same crash-loop risk fixed earlier for DATABASE_URL:
postinst's start guard only checked DATABASE_URL, so a fresh install
would now crash-loop on a missing ANTHROPIC_API_KEY instead. The guard
checks both. scripts/build.sh also now ships rules/*.yaml into the .deb
under /usr/share/deklarix/rules (not a conffile — rules are updated via
the release pipeline, never hand-edited on a server), and
deklarix.env.example points RULES_DIR there by default.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

internal/web/templates/index.html

@@ -0,0 +1,19 @@
+{{define "content"}}
+<h1>Pre-Publish-Prüfung</h1>
+<p>Caption und Plattform eingeben, um auf Kennzeichnungsrisiken zu prüfen.</p>
+
+<form hx-post="/pruefen" hx-target="#ergebnis" hx-swap="innerHTML">
+  <label for="platform">Plattform</label>
+  <select id="platform" name="platform" required>
+    <option value="instagram">Instagram</option>
+    <option value="tiktok">TikTok</option>
+  </select>
+
+  <label for="caption">Caption</label>
+  <textarea id="caption" name="caption" rows="6" required></textarea>
+
+  <button type="submit">Prüfen</button>
+</form>
+
+<div id="ergebnis"></div>
+{{end}}