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-compose.dev.yml

@@ -8,27 +8,35 @@ services:
     pull_policy: build
     container_name: normogen-backend-dev
     ports:
-      - '6500:8000'
+      # host 6501 (dev) -> container 6500, distinct from prod's 6500.
+      - '6501:6500'
     volumes:
       - ./src:/app/src
       - startup-logs:/tmp
     environment:
+      - APP_ENVIRONMENT=development
       - RUST_LOG=debug
-      - SERVER_PORT=8000
-      - SERVER_HOST=0.0.0.0
-      - MONGODB_URI=mongodb://mongodb:27017
-      - DATABASE_NAME=normogen_dev
-      - JWT_SECRET=dev-jwt-secret-key-minimum-32-chars
       - RUST_LOG_STYLE=always
+      - NORMOGEN_HOST=0.0.0.0
+      - NORMOGEN_PORT=6500
+      - MONGODB_URI=mongodb://mongodb:27017
+      - MONGODB_DATABASE=normogen_dev
+      - JWT_SECRET=dev-jwt-secret-key-minimum-32-chars
     depends_on:
       mongodb:
         condition: service_healthy
     networks:
       - normogen-network
+    healthcheck:
+      test: ["CMD", "curl", "-f", "http://localhost:6500/health"]
+      interval: 30s
+      timeout: 10s
+      retries: 3
+      start_period: 40s
     restart: unless-stopped
 
   mongodb:
-    image: mongo:6.0
+    image: mongo:7
     container_name: normogen-mongodb-dev
     ports:
       - '27017:27017'