docs: Reshape Database concepts, fold ID strategy, many-to-many, and migration gaps

[developerjamiu]

Part of the Concepts Phase 2 rollout (#671). Reshapes the Database concept group and folds in three open gaps.

Folds

• Add ID-strategy guidance: int vs UUID, autoincrement, client-set IDs #657 — adds a "Choosing an ID strategy" section to Database > Models: int-serial vs UuidValue trade-offs, serial as auto-increment, and the security implications of client-set UUIDs.
• Expand many-to-many docs: examples, why/when, and troubleshooting #652 — expands Many-to-many: when to reach for a junction table, "Managing junction records" (insertRow plus the attach/detach path), and a "Common errors" section.
• Improve migrations docs: custom SQL, Serverpod Cloud apply guidance, troubleshooting #659 — improves Migrations: a "Writing custom SQL" section, Serverpod Cloud apply guidance, and a "Troubleshooting" section.

Advances #504 (database docs epic).

Conformance

• Noun-first, unquoted description frontmatter across all 21 pages in the group (docs: Make authentication overview headings descriptive #635).
• House-style sweep on touched pages (banned vocab, backtick-start openings).

Not included

• Document database seeding (insert initial data at startup) #651 (seeding) — deferred; needs framework confirmation on the recommended startup entry point.
• One Add ID-strategy guidance: int vs UUID, autoincrement, client-set IDs #657 sub-example (a model carrying both a UuidValue id and a secondary serial int) is omitted: serial appears to be the primary-key default only, with no evidence it applies to a non-id field. Worth a framework-team check before documenting.
• Remaining serverpod generate references in the group are left for the dedicated modernization pass (Update references to 'serverpod generate' to reflect the changes with hot reload #538).

Notes for review

• Stacked on the Model reference PR (docs: Extract Model reference and reshape Models concept pages #674) — the base is concepts-phase2-pr3a-models, so the diff shows only this group's changes. Retarget to main once docs: Extract Model reference and reshape Models concept pages #674 merges.
• All technical claims verified against the framework main checkout; these are stable APIs, not 4.0-only behavior.
• Build passes locally.