refactor(storage): remove MinIO backend, filesystem-only storage

Remove the MinIO/S3 storage backend entirely. The filesystem backend is
fully implemented, already used in production, and a migrate-storage tool
exists for any remaining MinIO deployments to migrate beforehand.

Changes:
- Delete MinIO client implementation (internal/storage/storage.go)
- Delete migrate-storage tool (cmd/migrate-storage, scripts/migrate-storage.sh)
- Remove MinIO service, volumes, and env vars from all Docker Compose files
- Simplify StorageConfig: remove Endpoint, AccessKey, SecretKey, Bucket,
  UseSSL, Region fields; add SILO_STORAGE_ROOT_DIR env override
- Change all SQL COALESCE defaults from 'minio' to 'filesystem'
- Add migration 020 to update column defaults to 'filesystem'
- Remove minio-go/v7 dependency (go mod tidy)
- Update all config examples, setup scripts, docs, and tests

docs/MODULES.md

@@ -23,7 +23,7 @@ These cannot be disabled. They define what Silo *is*.
 |-----------|------|-------------|
 | `core` | Core PDM | Items, revisions, files, BOM, search, import/export, part number generation |
 | `schemas` | Schemas | Part numbering schema parsing, segment management, form descriptors |
-| `storage` | Storage | MinIO/S3 file storage, presigned uploads, versioning |
+| `storage` | Storage | Filesystem storage |
 
 ### 2.2 Optional Modules
 
@@ -470,12 +470,10 @@ Returns full config grouped by module with secrets redacted:
     "default": "kindred-rd"
   },
   "storage": {
-    "endpoint": "minio:9000",
-    "bucket": "silo-files",
-    "access_key": "****",
-    "secret_key": "****",
-    "use_ssl": false,
-    "region": "us-east-1",
+    "backend": "filesystem",
+    "filesystem": {
+      "root_dir": "/var/lib/silo/data"
+    },
     "status": "connected"
   },
   "database": {
@@ -566,7 +564,7 @@ Available for modules with external connections:
 
 | Module | Test Action |
 |--------|------------|
-| `storage` | Ping MinIO, verify bucket exists |
+| `storage` | Verify filesystem storage directory is accessible |
 | `auth` (ldap) | Attempt LDAP bind with configured credentials |
 | `auth` (oidc) | Fetch OIDC discovery document from issuer URL |
 | `odoo` | Attempt XML-RPC connection to Odoo |
@@ -602,11 +600,9 @@ database:
   sslmode: disable
 
 storage:
-  endpoint: minio:9000
-  bucket: silo-files
-  access_key: silominio
-  secret_key: silominiosecret
-  use_ssl: false
+  backend: filesystem
+  filesystem:
+    root_dir: /var/lib/silo/data
 
 schemas:
   directory: /etc/silo/schemas