feat: support custom memory types via MemoryType::Custom(String)

[allengaoo]

Summary

• Add Custom(String) variant to MemoryType enum, allowing downstream applications to define domain-specific memory categories (e.g. brand_theme, layout_catalog, writing_style) without modifying Memoria source code
• Make FromStr infallible — unrecognized strings are accepted as Custom(name) instead of returning InvalidMemoryType error (HTTP 422)
• Widen memory_type DB column from VARCHAR(20) to VARCHAR(64) to accommodate longer custom type names

Motivation

When integrating Memoria into a PPT generation agent, we needed domain-specific memory types (brand_theme, layout_catalog, writing_style, component_pattern) to store structured knowledge extracted from PowerPoint templates and style profiles. The current hardcoded 6-variant enum rejected these types with HTTP 422, forcing all custom semantics to be shoehorned into generic types like profile or semantic, which breaks type-filtered retrieval.

Changes

File	Change
memoria-core/src/types.rs	Add Custom(String) variant, hand-implement Serialize/Deserialize for flat JSON representation, make FromStr return Infallible, add is_builtin() helper
memoria-storage/src/store.rs	VARCHAR(20) → VARCHAR(64), update row_to_memory to use infallible parse
memoria-api/src/models.rs	Simplify parse_memory_type (now infallible)
memoria-api/src/routes/governance.rs	Update to infallible parse, remove unused import
memoria-mcp/src/tools.rs	Update two call sites to infallible parse

Backward Compatibility

• All 6 built-in types (semantic, working, episodic, profile, tool_result, procedural) parse to their original enum variants — zero behavioral change for existing data
• InvalidMemoryType error variant is retained in MemoriaError for backward compat but is no longer produced by FromStr
• JSON serialization format is unchanged for built-in types (flat strings)
• Custom types serialize as flat strings (e.g. "brand_theme", not {"Custom":"brand_theme"})
• VARCHAR(64) is backward compatible with existing VARCHAR(20) data

Test plan

• cargo test -p memoria-core — all 12 tests pass including new test_custom_type_roundtrip and test_custom_type_serde_roundtrip
• cargo build --release — zero warnings
• Manual API validation: POST /v1/memories with memory_type: "brand_theme" returns 200 (previously 422)
• Manual retrieval validation: POST /v1/memories/retrieve with memory_types: ["brand_theme"] returns filtered results