refactor: replace pdftoppm with go-fitz for in-process PDF rendering

Switch PDF page rendering from external pdftoppm/pdftocairo subprocess calls
to github.com/gen2brain/go-fitz (MuPDF wrapper), eliminating the poppler-utils
runtime dependency. Enable CGO in Dockerfile builder stage and install gcc/musl-dev
for the static MuPDF link; runtime image remains unchanged.

Dockerfile

@@ -10,9 +10,10 @@ ENV GOPROXY=https://goproxy.cn,direct
 ENV GOSUMDB=off
 
 
-# Build binary
-RUN go mod download && \ 
-    CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -o doc_ai ./main.go 
+# Build binary (CGO required for go-fitz/MuPDF)
+RUN apk add --no-cache gcc musl-dev && \
+    go mod download && \
+    CGO_ENABLED=1 GOOS=linux go build -ldflags="-s -w" -o doc_ai ./main.go
 
 # Runtime stage
 FROM crpi-8s2ierii2xan4klg.cn-beijing.personal.cr.aliyuncs.com/texpixel/alpine:latest