fix(backend): P2 config & Docker consistency

Resolve the operational/config sprawl (#10-#14 from the review): the app read
NORMOGEN_*/MONGODB_* env vars but every env/compose file set SERVER_*/DATABASE_*,
the ports were all over the place (8080/8000/6500/6800), there were 5
inconsistent Dockerfiles (rust:1.82 vs rust:1.93, missing curl), and an 18 MB
binary was committed.

Env-var names — standardize on what the code reads:
* config/mod.rs: NORMOGEN_PORT default 8080 -> 6500 (avoid the over-common
  8000/8080).
* db/mod.rs: create_database() now reads MONGODB_DATABASE (was DATABASE_NAME).
* .env.example, defaults.env, docker-compose.yml, docker-compose.dev.yml,
  DEPLOYMENT_GUIDE.md, deployment/README.md, deploy-and-test-solaria.sh,
  deploy-local-build.sh: use NORMOGEN_HOST/NORMOGEN_PORT/MONGODB_URI/
  MONGODB_DATABASE/APP_ENVIRONMENT; drop the dead SERVER_*/DATABASE_URI/
  DATABASE_NAME names.

Ports — canonical container port 6500 everywhere:
* Both Dockerfiles EXPOSE 6500; prod compose maps 6500:6500, dev 6501:6500.
* Bulk-replaced the long tail of solaria:8000/localhost:8000/localhost:8080 in
  docs and test scripts -> 6500.

Dockerfiles — 2 canonical, rust:latest, curl + healthcheck:
* backend/Dockerfile (prod): rust:latest builder, debian runtime now installs
  curl (so the compose HEALTHCHECK actually works), EXPOSE 6500.
* backend/docker/Dockerfile.dev (dev): rust:latest both stages, EXPOSE 6500.
* Deleted 3 redundant Dockerfiles (Dockerfile.improved x2, docker/Dockerfile).
* Deleted the committed 18 MB binary backend/docker/normogen-backend.
* Deleted 2 stray fix-notes in backend/docker/.

Compose:
* docker-compose.yml: correct env names, 6500:6500, APP_ENVIRONMENT=production,
  JWT_SECRET/ENCRYPTION_KEY required via compose interpolation, dropped the
  obsolete top-level version: key.
* docker-compose.dev.yml: correct env names, 6501:6500, mongo:7 (was 6.0),
  added a working backend healthcheck.
* Deleted docker/docker-compose.improved.yml + backend/deploy-to-solaria-improved.sh
  (built around the now-deleted 'improved' Docker files).

Verified: cargo fmt --check clean, build + clippy --all-targets clean, 18 unit
tests pass; grep confirms no SERVER_*/DATABASE_* env names and no rust:1.x tags
remain outside docs/archive and docs/adr (historical).

backend/docker/Dockerfile.dev

@@ -1,40 +1,35 @@
-# Development Dockerfile
-# Uses Rust 1.93+ to support Edition 2024 dependencies
-FROM rust:1.93-slim as builder
+# Development image. Built by docker-compose.dev.yml.
+# Debug build; the dev compose mounts ./src:/app/src for rapid rebuilds.
+FROM rust:latest AS builder
 
 WORKDIR /app
 
-# Install build dependencies
 RUN apt-get update && apt-get install -y \
     pkg-config \
     libssl-dev \
     && rm -rf /var/lib/apt/lists/*
 
-# Copy Cargo files first for better caching
+# Cache dependency build via a dummy main.rs
 COPY Cargo.toml ./
-
-# Create dummy main.rs for dependency caching
 RUN mkdir src && echo 'fn main() {}' > src/main.rs
-
-# Build dependencies (this layer will be cached)
 RUN cargo build && rm -rf src
 
-# Copy actual source code
+# Copy actual source and build
 COPY src ./src
-
-# Build the application
 RUN cargo build
 
-# Runtime stage
-FROM rust:1.93-slim
+# Runtime stage — full Rust image so the dev compose can rebuild in-container.
+# curl is required for the compose HEALTHCHECK to work.
+FROM rust:latest
+
+RUN apt-get update && apt-get install -y \
+    curl \
+    && rm -rf /var/lib/apt/lists/*
 
 WORKDIR /app
-
-# Copy the binary from builder
 COPY --from=builder /app/target/debug/normogen-backend /app/normogen-backend
 
-# Expose port
-EXPOSE 8000
+# The app listens on NORMOGEN_PORT (default 6500, set in compose/.env).
+EXPOSE 6500
 
-# Run the binary directly
 CMD ["/app/normogen-backend"]