feat(tenant): bill AI points by output characters

Previously AI point cost was computed from request (input) characters at
reservation time. Switch to reserving a single point up front and
settling the final cost from the generated output length on completion.

- Reserve 1 point (or FixedPoints) instead of pricing on request chars
- On completion, recompute points from output chars vs base chars,
  update the reservation amounts, and post a ledger delta for the
  top-up/refund; invalidate the workspace quota summary cache
- MarkCompleted now persists final request_chars/base_chars/points
- Use ceil division in calculateAIPointCost so an exact base boundary
  charges one point instead of rolling over
- Thread output text through all CompleteAIPoints callers (article
  selection, KOL assist, question expansion, template assist, compliance
  judge) and return the settled reservation
- Add unit tests plus an integration test gated on TEST_DATABASE_URL
- Update the user manual: billing is by output characters

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

server/internal/tenant/compliance/service_test.go

@@ -1,6 +1,7 @@
 package compliance
 
 import (
+	"strings"
 	"testing"
 
 	sharedcompliance "github.com/geo-platform/tenant-api/internal/shared/compliance"
@@ -34,3 +35,39 @@ func TestDecidePassesWithoutHits(t *testing.T) {
 		t.Fatalf("decide without hits = %s, want %s", got, sharedcompliance.GateDecisionPass)
 	}
 }
+
+func TestComplianceAIPointCostUsesOutputBoundary(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name      string
+		chars     int
+		baseChars int
+		want      int
+	}{
+		{name: "empty output minimum", chars: 0, baseChars: 1000, want: 1},
+		{name: "single char", chars: 1, baseChars: 1000, want: 1},
+		{name: "exactly one base", chars: 1000, baseChars: 1000, want: 1},
+		{name: "one over base", chars: 1001, baseChars: 1000, want: 2},
+		{name: "default base", chars: 2000, baseChars: 0, want: 2},
+	}
+
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+			if got := calculateAIPointCost(tt.chars, tt.baseChars); got != tt.want {
+				t.Fatalf("calculateAIPointCost(%d, %d) = %d, want %d", tt.chars, tt.baseChars, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestComplianceAIPointCharsTrimWhitespace(t *testing.T) {
+	t.Parallel()
+
+	got := countAIPointChars("  " + strings.Repeat("字", 1001) + "\n")
+	if got != 1001 {
+		t.Fatalf("countAIPointChars = %d, want 1001", got)
+	}
+}