Fix type mismatches for nullable dimensions and context_limit fields

[Copilot]

Migration 004 made dimensions and context_limit nullable in the definitions table, but generated models still used int32 instead of pgtype.Int4, causing compilation errors when assigning values.

Changes

• Migration: Added 005_make_dimensions_nullable.sql to make instances.dimensions nullable (was only done for definitions)
• Models: Regenerated with sqlc to reflect nullable schema: dimensions and context_limit are now pgtype.Int4 in both Definition and Instance structs
• Type conversions: Updated 8 locations across handlers to properly convert between API layer (int32) and database layer (pgtype.Int4)
• Null safety: Added validation checks before accessing .Int32 on nullable fields in critical paths (embedding validation, similarity search)

Pattern

// Before (compilation error)
Dimensions: int32(input.Body.Dimensions)

// After (nullable-aware)
Dimensions: pgtype.Int4{Int32: int32(input.Body.Dimensions), Valid: input.Body.Dimensions != 0}

// Reading back (with null check where required)
if !instance.Dimensions.Valid {
    return huma.Error500InternalServerError("dimensions not configured")
}
dimension := instance.Dimensions.Int32

Enables database migration from v003 schemas where these fields may be unset.

Original prompt

With the current code, there are errors in compiling:

# github.com/mpilhlt/embapi/internal/handlers
internal/handlers/llm_services.go:77:22: cannot use int32(input.Body.Dimensions) (value of type int32) as pgtype.Int4 value in struct literal
internal/handlers/llm_services.go:78:22: cannot use int32(input.Body.ContextLimit) (value of type int32) as pgtype.Int4 value in struct literal
internal/handlers/llm_services.go:180:21: cannot use def.Dimensions (variable of struct type pgtype.Int4) as int32 value in struct literal
internal/handlers/llm_services.go:181:21: cannot use def.ContextLimit (variable of struct type pgtype.Int4) as int32 value in struct literal
internal/handlers/llm_services.go:595:28: cannot use definition.Dimensions (variable of struct type pgtype.Int4) as int32 value in assignment
internal/handlers/llm_services.go:598:30: cannot use definition.ContextLimit (variable of struct type pgtype.Int4) as int32 value in assignment
internal/handlers/llm_services.go:612:21: cannot use int32(input.Body.ContextLimit) (value of type int32) as pgtype.Int4 value in struct literal
internal/handlers/llm_services.go:719:17: cannot use llm.ContextLimit (variable of struct type pgtype.Int4) as int32 value in struct literal

I need the dimensions and context_limit fields to be nullable (because I want to migrate existing databases from an earlier versions that might not have these fields), but that produces these errors. Please either fix the errors or add a function to the migration script that takes care of unset fields. Thank you.

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[awagner-mainz]

Code looks good, tests pass.