feat: multi-caliber firearms (firearm_calibers join table)

[crzykidd]

Roadmap item from PRD §10.8. Some firearms fire multiple calibers/conversions; v0.3.0 firearms have a single caliber_id plus free-text caliber_notes.

Scope

• Add a firearm_calibers join table to associate a firearm with more than one caliber.
• Do not rename the existing caliber_id column — add the join table additively in a new migration; caliber_id can remain the primary/default caliber.
• UI to add/remove additional calibers on a firearm.

Explicitly out of scope

• A per-model alternative-caliber catalog was explicitly rejected in the PRD. Users continue recording informal compatibility (e.g. ".357 Magnum chamber, also fires .38 Special") in caliber_notes. This issue is only about associating real calibers a user owns/shoots to a firearm.

Source: PRD §10.8 and §6.7.2.