chore(users): remove coordinator leftovers, tidy for userservice

- delete api/requests.http and ARCHITECTURE.md (coordinator content)
- rewrite README.md and .env.example for the userservice
- drop dead RunPeriodic (reaper machinery userservice has no use for)
- fix .gitignore/.dockerignore/.golangci.yml module + artifact names
- degeneralize stale copied comments that said "coordinator"

users/.env.example

@@ -1,32 +1,23 @@
 # Copy to .env and adjust. All settings are read from the environment.
 
-COORDINATOR_ADDR=:8080
-DATABASE_URL=postgres://scimesh:scimesh@localhost:5432/scimesh?sslmode=disable
+USERSERVICE_ADDR=:8081
+DATABASE_URL=postgres://scimesh:scimesh@localhost:5433/scimesh_users?sslmode=disable
 
-# Shared bearer token every worker must present. Leave empty to disable auth (dev only).
-WORKER_AUTH_TOKEN=change-me
-
-# Optional local operator UI. Use a separate value; never reuse the worker token.
-# When empty, /ui is disabled.
-UI_AUTH_TOKEN=
+# Shared HS256 secret used to sign JWTs. The coordinator verifies tokens with
+# this SAME secret, so the two values must match exactly. Minimum 32 bytes.
+JWT_SECRET=change-me-to-a-long-random-secret-min-32-bytes
+# How long an issued token stays valid.
+JWT_TTL=24h
+# bcrypt work factor. Empty/0 uses the library default (10).
+# BCRYPT_COST=10
 
 # Logging. LOG_LEVEL: debug|info|warn|error. LOG_FILE empty = stdout only;
 # set a path to also write a size-rotated file (kept across restarts).
 LOG_LEVEL=info
-# LOG_FILE=./logs/coordinator.log
-
-# Directory where artifact bytes are stored.
-COORDINATOR_STORAGE_DIR=./data
-# Upper bound on an uploaded dataset or artifact body (bytes). Default 1 GiB.
-MAX_UPLOAD_BYTES=1073741824
+# LOG_FILE=./logs/userservice.log
 
 # Optional tuning (defaults shown).
 DB_MAX_CONNS=10
 # How long to keep retrying the initial DB connection while Postgres boots.
 DB_CONNECT_TIMEOUT=30s
 REQUEST_TIMEOUT=15s
-LEASE_DURATION=2m
-DEFAULT_MAX_ATTEMPTS=3
-REAPER_INTERVAL=30s
-# A worker silent longer than this is marked offline by the reaper.
-WORKER_OFFLINE_AFTER=1m