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/src/config/mod.rs

@@ -147,7 +147,7 @@ impl Config {
             server: ServerConfig {
                 host: std::env::var("NORMOGEN_HOST").unwrap_or_else(|_| "0.0.0.0".to_string()),
                 port: std::env::var("NORMOGEN_PORT")
-                    .unwrap_or_else(|_| "8080".to_string())
+                    .unwrap_or_else(|_| "6500".to_string())
                     .parse()?,
             },
             database: DatabaseConfig {

backend/src/db/mod.rs

@@ -23,7 +23,7 @@ pub use mongodb_impl::MongoDb;
 
 pub async fn create_database() -> Result<Database> {
     let mongo_uri = env::var("MONGODB_URI").expect("MONGODB_URI must be set");
-    let db_name = env::var("DATABASE_NAME").expect("DATABASE_NAME must be set");
+    let db_name = env::var("MONGODB_DATABASE").expect("MONGODB_DATABASE must be set");
 
     let client = Client::with_uri_str(&mongo_uri).await?;
     let database = client.database(&db_name);