Add TCP keepalive options for database connections

[tsg]

This adds the following config options. These are set on the underlying TCP socket.

• ELECTRIC_DATABASE_TCP_KEEPALIVE_IDLE - Idle time before probing. Enables SO_KEEPALIVE
• ELECTRIC_DATABASE_TCP_KEEPALIVE_INTERVAL - Gap between probes.
• ELECTRIC_DATABASE_TCP_KEEPALIVE_COUNT - Unanswered probes before drop.
• ELECTRIC_DATABASE_TCP_USER_TIMEOUT - Max time data may stay unacknowledged.

All unset by default, so behaviour is unchanged unless configured. Durations accept the usual 30s / 500ms forms.

Sample config:

ELECTRIC_DATABASE_TCP_KEEPALIVE_IDLE=30s
ELECTRIC_DATABASE_TCP_KEEPALIVE_INTERVAL=10s
ELECTRIC_DATABASE_TCP_KEEPALIVE_COUNT=3
ELECTRIC_DATABASE_TCP_USER_TIMEOUT=60s

Implementation:

ConnectionResolver.populate_tcp_opts/1 now appends the options to :socket_options. SO_KEEPALIVE goes through :inet's portable {:keepalive, true}; the rest use :raw with the Linux IPPROTO_TCP option numbers, guarded on :os.type() so non-Linux developer machines skip them rather than failing with einval.

Note: I've generated the PR with AI, because I don't know Elixir, but the change looks fairly straightforward to me, so I'm hoping it's all correct. The one thing that feels a bit risky is what happens on non-Linux kernels. The code attempts to disable this automatically in that case.

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