Guard concurrent sends with exclusive DB lock and URI/RK checks

[Mshehu5]

This PR Closes #1032

As discussed in the issue two concurrent send commands targeting the same URI could select different coins. PRAGMA locking_mode = EXCLUSIVE has been set as stated to make only one process holds write access at a time.

And also check to make sure URI and recceiver pubkey which are in use for another session is not reused

Co-authored by claude sonnet 4.6

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[coveralls]

Pull Request Test Coverage Report for Build 22582242969

Details

• 63 of 70 (90.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 82.526%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
payjoin-cli/src/app/v2/mod.rs	0	1	0.0%
payjoin-cli/src/db/v2.rs	60	61	98.36%
payjoin-cli/src/db/error.rs	0	5	0.0%

Totals
Change from base Build 22579575348:	0.04%
Covered Lines:	10683
Relevant Lines:	12945

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💛 - Coveralls