[plainprogrammer]

Prepares main for tagging v0.3.0 (docs/releasing.md, Cutting a release, step 1).

Upgrade proof seeds (spec 031)

• script/upgrade/seeds/next.rb → script/upgrade/seeds/0.3.0.rb, with the released-seed header (as 0.2.1.rb has)
• script/upgrade/seed_exceptions.yml gains a "0.3.0": [] key
• A new next.rb starts from the same content; nothing is pending for the release after 0.3.0 yet

After the tag, CI's upgrade proof upgrades from v0.3.0 using 0.3.0.rb.

Upgrade notes

v0.2.1 had no upgrade notes, and several migrations since then had none either. This adds:

• Decks (spec 062): what AddDecks changes on locations and the move to export format 4
• Instance settings one row (Hold instance_settings to exactly one row #229)
• Session idle expiry (spec 049): the IP address and user agent are dropped on purpose; nobody is signed out by the upgrade
• Each account's currency (spec 038): set COLLECTOR_CURRENCY before upgrading; an unsupported code fails the migration
• Smaller additions: additive tables and columns only (specs 015, 026, 032, 037, 040, 042)

Verification

• ruby script/upgrade/guard.rb: v0.2.1's 14 migrations are unchanged
• bin/rspec spec/script/upgrade: 136 examples, 0 failures
• The pre-push bin/ci passed on c8f7afe

After merge

1. Tag v0.3.0, annotated, on the merge commit, and push the tag
2. Watch the release run and check both architectures and the upgrade smoke steps
3. Delete the two sessions entries (migration 20261109100003) from script/upgrade/expected_changes.yml, because v0.3.0 contains that migration (step 7)

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