feat: Make TTL configurable for ExternalDNS integration

[Claude]

Original prompt

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This section details on the original issue you should resolve

<issue_title>Feature Request: Make TTL configurable</issue_title>
<issue_description>Hello,

Im using desec for my DNS and they have a minimum TTL of 3600. In the code, I can see that the TTL is set to 300 and is not configurable (as far as I can tell anyway, I've never properly worked with Go).

I think it would be good to make this configurable. I'd like to have a go at this and just wanted to write down my plan of attack:

• Update config.go, add "ttl" to ExternalDNSIntegrationConfig, make it optional
• Update externaldns.go, change NewExternalDNS so we can pass in a value for ttl. If the value passed in is nil, set to our default ttl here
• Update utils.go, there are two references to NewExternalDns. Pass the new ttl value in.

There may be other instances I have to change - Ill find out once I actually fork the repo. But keen to hear if you think this is a sensible way to do it.</issue_description>

Comments on the Issue (you are @claude[agent] in this section)

• Fixes Feature Request: Make TTL configurable #215

[borchero]

@claude[agent] run pixi run pre-commit run --all-files

[Claude]

@claude[agent] run pixi run pre-commit run --all-files

Pre-commit checks completed. Fixed Go formatting issue in d276acf.

Since pixi is not available in this environment, I manually ran the equivalent checks:

• ✅ No trailing whitespace
• ✅ Proper end-of-file newlines
• ✅ Go formatting (gofmt) - fixed extra blank line
• ✅ Helm chart lints successfully
• ✅ Code compiles and tests pass