v0.1.16 Relicense to AGPL-3.0-only + flow-ingestion hardening

📋 Full release notes: RELEASE_NOTES.md

Licensing and flow-ingestion hardening. Yagra is now released under the GNU Affero General
Public License v3.0 (AGPL-3.0-only), and this release hardens the traffic-flow ingestion path so a
busy or slow flow store can't disturb the rest of the system. There are no new user-facing features —
a default install behaves exactly as v0.1.15 did, now under the new license.

Licensing

• Relicensed to AGPL-3.0-only. Every source file now carries an SPDX header and the repository
  ships under AGPL-3.0-only. Because Yagra is typically run as a network service, note AGPL §13:
  if you run a modified version and let others use it over a network, you must offer them its source.
  For terms other than the AGPL (e.g. embedding Yagra in a proprietary product), a commercial
  license may be available — see the README.

Improvements

• Flow ingestion is isolated from a slow flow store. The ClickHouse flow writer now runs
  separately from the flow bus consumer, handing rows off over a bounded queue. A slow or hung
  ClickHouse can no longer back the yagra.flows subscription up into a silent message drop — under
  pressure, flow rows are dropped and counted (the store is loss-tolerant by design) instead of
  stalling ingestion.
• Flow exporter resolution no longer reloads the node table per batch. Flow from an exporter whose
  source IP isn't a registered node (common for routers that export from a loopback) used to re-scan
  the whole node table on every batch; each unmapped exporter is now retried at most once, so flow
  ingestion stays cheap at tens of thousands of nodes.

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Container images (published by CI to GHCR — pull :v0.1.16 for this release):

• ghcr.io/horryworks/yagra-core:v0.1.16
• ghcr.io/horryworks/yagra-poller:v0.1.16
• ghcr.io/horryworks/yagra-web:v0.1.16