feat: AI-powered features — document drafting, case strategy, similar case finder (P2)

Backend:
- DraftDocument: Claude generates legal documents from case data + template type
  (14 template types: Klageschrift, UPC claims, Abmahnung, etc.)
- CaseStrategy: Opus-powered strategic analysis with next steps, risk assessment,
  and timeline optimization (structured tool output)
- FindSimilarCases: queries youpc.org Supabase for UPC cases, Claude ranks by
  relevance with explanations and key holdings

Endpoints: POST /api/ai/draft-document, /case-strategy, /similar-cases
All rate-limited (5 req/min) and permission-gated (PermAIExtraction).
YouPC database connection is optional (YOUPC_DATABASE_URL env var).

backend/internal/handlers/ai.go

@@ -5,6 +5,8 @@ import (
 	"io"
 	"net/http"
 
+	"github.com/google/uuid"
+
 	"mgit.msbls.de/m/KanzlAI-mGMT/internal/auth"
 	"mgit.msbls.de/m/KanzlAI-mGMT/internal/services"
 )
@@ -115,3 +117,139 @@ func (h *AIHandler) SummarizeCase(w http.ResponseWriter, r *http.Request) {
 		"summary": summary,
 	})
 }
+
+// DraftDocument handles POST /api/ai/draft-document
+// Accepts JSON {"case_id": "uuid", "template_type": "string", "instructions": "string", "language": "de|en|fr"}.
+func (h *AIHandler) DraftDocument(w http.ResponseWriter, r *http.Request) {
+	tenantID, ok := auth.TenantFromContext(r.Context())
+	if !ok {
+		writeError(w, http.StatusForbidden, "missing tenant")
+		return
+	}
+
+	var body struct {
+		CaseID       string `json:"case_id"`
+		TemplateType string `json:"template_type"`
+		Instructions string `json:"instructions"`
+		Language     string `json:"language"`
+	}
+	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+
+	if body.CaseID == "" {
+		writeError(w, http.StatusBadRequest, "case_id is required")
+		return
+	}
+	if body.TemplateType == "" {
+		writeError(w, http.StatusBadRequest, "template_type is required")
+		return
+	}
+
+	caseID, err := parseUUID(body.CaseID)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "invalid case_id")
+		return
+	}
+
+	if len(body.Instructions) > maxDescriptionLen {
+		writeError(w, http.StatusBadRequest, "instructions exceeds maximum length")
+		return
+	}
+
+	draft, err := h.ai.DraftDocument(r.Context(), tenantID, caseID, body.TemplateType, body.Instructions, body.Language)
+	if err != nil {
+		internalError(w, "AI document drafting failed", err)
+		return
+	}
+
+	writeJSON(w, http.StatusOK, draft)
+}
+
+// CaseStrategy handles POST /api/ai/case-strategy
+// Accepts JSON {"case_id": "uuid"}.
+func (h *AIHandler) CaseStrategy(w http.ResponseWriter, r *http.Request) {
+	tenantID, ok := auth.TenantFromContext(r.Context())
+	if !ok {
+		writeError(w, http.StatusForbidden, "missing tenant")
+		return
+	}
+
+	var body struct {
+		CaseID string `json:"case_id"`
+	}
+	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+
+	if body.CaseID == "" {
+		writeError(w, http.StatusBadRequest, "case_id is required")
+		return
+	}
+
+	caseID, err := parseUUID(body.CaseID)
+	if err != nil {
+		writeError(w, http.StatusBadRequest, "invalid case_id")
+		return
+	}
+
+	strategy, err := h.ai.CaseStrategy(r.Context(), tenantID, caseID)
+	if err != nil {
+		internalError(w, "AI case strategy analysis failed", err)
+		return
+	}
+
+	writeJSON(w, http.StatusOK, strategy)
+}
+
+// SimilarCases handles POST /api/ai/similar-cases
+// Accepts JSON {"case_id": "uuid", "description": "string"}.
+func (h *AIHandler) SimilarCases(w http.ResponseWriter, r *http.Request) {
+	tenantID, ok := auth.TenantFromContext(r.Context())
+	if !ok {
+		writeError(w, http.StatusForbidden, "missing tenant")
+		return
+	}
+
+	var body struct {
+		CaseID      string `json:"case_id"`
+		Description string `json:"description"`
+	}
+	if err := json.NewDecoder(r.Body).Decode(&body); err != nil {
+		writeError(w, http.StatusBadRequest, "invalid request body")
+		return
+	}
+
+	if body.CaseID == "" && body.Description == "" {
+		writeError(w, http.StatusBadRequest, "either case_id or description is required")
+		return
+	}
+
+	if len(body.Description) > maxDescriptionLen {
+		writeError(w, http.StatusBadRequest, "description exceeds maximum length")
+		return
+	}
+
+	var caseID uuid.UUID
+	if body.CaseID != "" {
+		var err error
+		caseID, err = parseUUID(body.CaseID)
+		if err != nil {
+			writeError(w, http.StatusBadRequest, "invalid case_id")
+			return
+		}
+	}
+
+	cases, err := h.ai.FindSimilarCases(r.Context(), tenantID, caseID, body.Description)
+	if err != nil {
+		internalError(w, "AI similar case search failed", err)
+		return
+	}
+
+	writeJSON(w, http.StatusOK, map[string]any{
+		"cases": cases,
+		"count": len(cases),
+	})
+}