docs: add and refine project documentation

Signed-off-by: Marcus Fihlon <marcus@fihlon.swiss>

.env.example

@@ -1,13 +1,46 @@
-# FitPub Environment Configuration
+# FitPub environment configuration
+#
+# This file mirrors the environment variables referenced by the Spring
+# configuration and uses the same effective defaults as the development profile
+# where defaults exist.
 
-# Registration Settings
-# Leave empty for open registration, or set a password to require it for new signups
-REGISTRATION_PASSWORD=
+# Spring profile
+# Use `prod` for production deployments.
+# SPRING_PROFILES_ACTIVE=dev
 
-# Example with password (uncomment to enable):
-# REGISTRATION_PASSWORD=my-secret-invite-code-2024
+# Local PostgreSQL / PostGIS
+# SPRING_DATASOURCE_URL=jdbc:postgresql://localhost:5432/fitpub
+# SPRING_DATASOURCE_USERNAME=fitpub
+# SPRING_DATASOURCE_PASSWORD=change_me_in_production
 
-# Other settings
-# REGISTRATION_ENABLED=true
+# Server
+# PORT=8080
+
+# Public FitPub URLs
 # FITPUB_DOMAIN=localhost:8080
 # FITPUB_BASE_URL=http://localhost:8080
+
+# ActivityPub
+# FITPUB_ALLOW_PRIVATE_IPS=false
+# FITPUB_FEDERATION_PROTOCOL=https
+
+# Authentication
+# JWT_SECRET=dev-secret-key-change-in-production-must-be-at-least-32-characters-long
+
+# Registration
+# REGISTRATION_ENABLED=true
+# Leave empty for open registration, or set a password to require it for new signups
+# REGISTRATION_PASSWORD=
+
+# Storage
+# Application default is ${java.io.tmpdir}/fitpub/images
+# FITPUB_IMAGES_PATH=/tmp/fitpub/images
+# Application default is ${java.io.tmpdir}/fitpub/tiles
+# FITPUB_TILE_CACHE_PATH=/tmp/fitpub/tiles
+
+# Image generation
+# OSM_TILES_ENABLED=true
+
+# Weather
+# WEATHER_ENABLED=false
+# OPENWEATHERMAP_API_KEY=