Validate config at startup to prevent ticker panic

[LarsLaskowski]

Summary

internal/config/config.go performed no validation after merging defaults,
YAML, and flags, so broken values reached the runtime unchecked. Most
seriously, poll_interval_seconds: 0 (or negative) produced a 0-duration
time.NewTicker in Collector.Run, which panics and crashes the whole
service — a single typo in /etc/pimonitor/config.yaml took the daemon down
in a Restart=on-failure loop. Other broken values were silently misapplied
(empty listen_addr → port 80, unknown log_level → info, negative
history_window_minutes → broken sparklines, inverted thresholds).

This change makes the daemon fail fast at startup with a clear message:

• Add Config.Validate() checking poll_interval_seconds > 0,
  updates_check_minutes > 0, updates_stale_threshold_minutes >= 0,
  history_window_minutes > 0, non-empty listen_addr, log_level ∈
  {debug, info, warn, error}, and all thresholds >= 0 with each warn <=
  its crit counterpart. Returns one descriptive error per first violation.
• Call cfg.Validate() at the end of Load() (after flag overrides). A
  -version request short-circuits before validation so pimonitor -version
  still works against an otherwise invalid config. run() already wraps the
  error as load config: ....
• As defense in depth, clamp non-positive tick intervals to 1s with a warning
  in Collector.Run so no future caller can trigger the ticker panic.

Verified by building the binary and running it against
poll_interval_seconds: 0:

pimonitor: load config: poll_interval_seconds must be > 0 (got 0)  (exit 1)

No panic, no stack trace.

Related Issue

Closes #25

Checklist

• Tests added/updated for the change (go test ./... passes locally) —
  table-driven tests in internal/config/config_test.go cover each
  rejected field, valid edge cases (warn == crit, zero stale threshold),
  that Default() passes Validate(), that Load rejects
  poll_interval_seconds: 0, and that -version bypasses validation.
  go test ./... -race passes.
• go vet ./... and golangci-lint run are clean (0 issues)
• Documentation updated if this changes the REST API (docs/API.md),
  configuration (README.md, packaging/pimonitor.example.yaml), or
  installation/packaging (packaging/install.sh, systemd units) — n/a,
  no config keys, defaults, or API shapes changed
• No breaking change to /api/v1/... response shapes, or a new API
  version was introduced instead