feat: replace Claude-based extraction with a rule-based engine

Deklarix itself no longer depends on the Anthropic API — that was a
separate API key/billing relationship from Claude Code (used to develop
Deklarix), which the user did not intend to take on for the product
itself.

Consideration (Gegenleistung) is no longer guessed from text — it's a
required form field now, since only the submitter actually knows
whether a business relationship existed. A keyword-only system can't
tell a covertly-paid post from a genuinely organic one; they read
identically. What internal/extract *can* still determine reliably and
deterministically from the caption: whether a disclosure keyword is
present (werbung, anzeige, bezahlte partnerschaft, paid partnership,
#ad, #werbung, #anzeige, #sponsored, #sponsoredby, #sponsoredpost —
case-insensitive), its exact original-case wording, and whether it sits
before the platform's "mehr anzeigen" truncation point (~125 chars
Instagram, ~150 TikTok — rough estimates, platforms change these without
notice, verify before real customer use).

internal/extract's Anthropic HTTP client and tool-use schema are gone
(client.go/api.go deleted), replaced by engine.go — a stateless Engine
with no network calls. extract.Result/ParsePayload keep the exact same
JSON shape as before (gegenleistung/kennzeichnung_vorhanden/
kennzeichnung_wortlaut/kennzeichnung_vor_kuerzung), so internal/store and
internal/dossier needed no changes at all — only extract itself, the web
form/handler (new consideration field), and main.go (no more
ANTHROPIC_API_KEY requirement) changed.

Trade-off the user was told and accepted: without an LLM, the system can
no longer independently catch undisclosed paid content that carries no
recognizable keyword at all — that now rests on the submitter's honesty.
Creative or implicit disclosure phrasing outside the keyword list also
won't be recognized.

Verified against a real running instance with zero API keys configured:
register -> check (real rule engine, correctly triggered WK-004 for a
disclosure placed 130 characters in, past the Instagram threshold) ->
archive -> PDF dossier download, all against real Postgres.

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

internal/web/templates/index.html

@@ -21,6 +21,19 @@
     <option value="video">Video</option>
   </select>
 
+  <label for="consideration">Gegenleistung</label>
+  <select id="consideration" name="consideration" required>
+    <option value="">— bitte wählen —</option>
+    <option value="bezahlt">Bezahlt</option>
+    <option value="sachbezug">Sachbezug (Produkt, Einladung, ...)</option>
+    <option value="keine">Keine</option>
+    <option value="unklar">Unklar</option>
+  </select>
+  <p class="hinweis">
+    Das kann die Prüfung nicht aus der Caption erraten — nur wer
+    einreicht, weiß, ob eine Gegenleistung vorlag.
+  </p>
+
   <label for="caption">Caption</label>
   <textarea id="caption" name="caption" rows="6" required></textarea>