feat: add optional generate_id/0 callback to kura_schema

[Taure]

Summary

• Adds generate_id/0 as an optional callback on kura_schema
• On insert, if no PK is provided, Kura checks for this callback before falling back to UUID v4 / BIGSERIAL
• Keeps Kura dependency-free — users bring their own ID generator (e.g. jhn_uuid:gen(v7, [binary]))

Test plan

• Callback is used on insert when no ID provided
• Explicit ID in changeset takes precedence over callback
• Each insert produces a unique generated ID
• Schemas without the callback behave unchanged (BIGSERIAL / UUID v4)
• All 155 existing tests pass

[github-actions]

🟢 Code Coverage — 93.4%

1916 of 2052 lines covered.

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🟡 ELP Lint — 1 warning

5 diagnostics found. See job logs for details.

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ℹ️ 11 OTP CVEs auto-ignored (already fixed in running version)

These CVEs are patched in the installed OTP version but NVD data
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scan via an auto-generated .trivyignore.

CVE	Details
CVE-2026-23943	Fixed in 28.4.1, running 28.4.1 — Pre-auth SSH DoS via unbounded zlib inflate
CVE-2026-23942	Fixed in 28.4.1, running 28.4.1 — SFTP root escape via component-agnostic prefix check in ssh_sftpd
CVE-2026-23941	Fixed in 28.4.1, running 28.4.1 — Request smuggling via first-wins Content-Length parsing in inets httpd
CVE-2026-21620	Fixed in 28.3.2, running 28.4.1 — TFTP Path Traversal
CVE-2016-1000107	Fixed in 28.0.4, running 28.4.1 — Httpd CGI Scripts Environment Variable Pollution AKA "httpoxy"
CVE-2025-58050	Fixed in 28.0.3, running 28.4.1 — Buffer Read Overflow on Regular Expressions with (*scs:) and (*ACCEPT)
CVE-2025-48038	Fixed in 28.0.3, running 28.4.1 — SSH Unverified File Handles can Cause Excessive Use of System Resources
CVE-2025-48039	Fixed in 28.0.3, running 28.4.1 — SSH Unverified Paths can Cause Excessive Use of System Resources
CVE-2025-48040	Fixed in 28.0.3, running 28.4.1 — SSH Malicious Key Exchange Messages may Lead to Excessive Resource Consumption
CVE-2025-48041	Fixed in 28.0.3, running 28.4.1 — SSH_FXP_OPENDIR may Lead to Exhaustion of File Handles
CVE-2025-4748	Fixed in 28.0.1, running 28.4.1 — Absolute Path in Zip Module