feat: auto disk usage limit mode

[pcfreak30]

What this does

Adds an automatic disk usage limit mode using real-time disk capacity via gopsutil. The setting is now a string with three modes:

Value	Behavior
auto	80% of total disk capacity (default)
0	Unlimited (no limit)
N	Fixed limit in gigabytes (e.g. 100 for 100 GB)

Why a string type

The old uint64 type could only express a fixed byte limit or 0 (unlimited). Adding auto required a sentinel value or a separate flag. Using a string keeps the config surface minimal and self-describing.

Backward compatibility

• Existing configs with YAML numbers (disk_usage_limit: 100 or 100) continue to work via a mapstructure decode hook.
• Environment variable overrides work unchanged.
• Empty/missing defaults to auto.

Files changed

• internal/config/config.go/config_test.go — DiskUsageLimit type, auto resolution, tests
• internal/backend/factory.go — wiring to s3d factory
• internal/handlers/handlers.go, handlers_config.go — API type/validation
• internal/views/settings.templ — three-mode UI selector
• web/src/pages/settings.ts, utils.ts — frontend state management
• go.mod/go.sum — gopsutil/v4 dependency

[kody-ai]

Found critical issues please review the requested changes

• Auto disk-limit mode silently disables the limit (unlimited) when free space is below 80GB, the scenario most needing protection.

[kody-ai]

Found critical issues please review the requested changes

• Auto disk-limit default makes backend startup hard-fail when gopsutil disk.Usage errors (new failure surface vs old unlimited default).

[kody-ai]

Kody Review Complete

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Keep up the excellent work! 🚀

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[github-actions]

Docker image for testing

Pull the image for this PR:

docker pull ghcr.io/lumeweb/s3-server:sha-68f3c67

Or use in docker-compose:

services:
  s3-server:
    image: ghcr.io/lumeweb/s3-server:sha-68f3c67

The image is rebuilt on every push to this PR's branch.