docs: add production deployment example with volumes and .env

- docker/docker-compose.prod.yml: pulls image from 192.168.0.19:5000,
  mounts config/, wiki/ and db/ as separate volumes, health check, TZ support
- .env.example: fully commented template for all env vars
- .gitignore: ignore .env, always track .env.example
- README: deployment section with step-by-step guide, volume table, backup commands
- config.go: default db_path to /data/db/archivum.db (matches prod volume layout)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.example

@@ -0,0 +1,51 @@
+# ── Archivum — environment variables example ──────────────────────────────────
+# Copy to .env and edit before running docker-compose.prod.yml
+#
+#   cp .env.example .env
+#
+# Lines starting with # are comments and are ignored.
+# ──────────────────────────────────────────────────────────────────────────────
+
+
+# ── Registry ───────────────────────────────────────────────────────────────────
+# Address of your private Docker registry.
+REGISTRY=192.168.0.19:5000
+
+# Image tag to pull. Use a specific digest in production for reproducible deploys.
+#   IMAGE_TAG=sha256:abc123...
+IMAGE_TAG=latest
+
+
+# ── Host paths ─────────────────────────────────────────────────────────────────
+# Base directory — all data lives under this path.
+# Adjust to wherever you want to store Archivum data on the host.
+ARCHIVUM_BASE=/opt/archivum
+
+# Configuration directory (config.json, settings.json).
+# The Setup Wizard will create config.json here on first access.
+ARCHIVUM_CONFIG=/opt/archivum/config
+
+# AsciiDoc wiki files + git repository root.
+ARCHIVUM_WIKI=/opt/archivum/wiki
+
+# SQLite database directory.
+# The database file will be created as archivum.db inside this directory.
+ARCHIVUM_DB=/opt/archivum/db
+
+
+# ── Network ────────────────────────────────────────────────────────────────────
+# Port exposed on the host. Access the app at http://<host>:HOST_PORT
+HOST_PORT=8080
+
+
+# ── File system permissions ─────────────────────────────────────────────────────
+# UID and GID of the user that owns the host directories above.
+# Run `id` on the host to find the correct values.
+# Using 1000:1000 is fine if you created the directories as your own user.
+PUID=1000
+PGID=1000
+
+
+# ── Timezone ───────────────────────────────────────────────────────────────────
+# Used for git commit timestamps. See https://en.wikipedia.org/wiki/List_of_tz_database_time_zones
+TZ=Europe/Stockholm