Fix: Pin cargo-watch to v8.4.0 to avoid edition2024 requirement

Problem:
- cargo-watch v8.5.3 requires Rust edition2024 which is not stable yet
- Even Rust 1.83 doesn't support edition2024
- Build fails with: feature 'edition2024' is required

Solution:
- Pin cargo-watch to version 8.4.0
- This version works perfectly with stable Rust 1.83
- No functional difference for development purposes

Change:
RUN cargo install cargo-watch
→ RUN cargo install cargo-watch --version 8.4.0

backend/docker/Dockerfile.dev

@@ -1,7 +1,8 @@
 FROM rust:1.83-alpine
 WORKDIR /app
 RUN apk add --no-cache musl-dev pkgconf openssl-dev curl wget git pkgconfig
-RUN cargo install cargo-watch
+# Pin cargo-watch to version 8.4.0 which doesn't require edition2024
+RUN cargo install cargo-watch --version 8.4.0
 COPY Cargo.toml ./
 RUN mkdir src && echo 'fn main() {}' > src/main.rs
 RUN cargo build && rm -rf src