REST API OpenAPI spec (#612)

* OpenAPI spec in specs/api.  Checked lint with redoc.

specs/api/components/schemas/common/DocumentMetadata.yaml

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+type: object
+description: Document metadata for library management
+properties:
+  url:
+    type: string
+    description: Document URL or identifier
+    example: https://example.com/document.pdf
+  title:
+    type: string
+    description: Document title
+    example: Example Document
+  author:
+    type: string
+    description: Document author
+    example: John Doe
+  date:
+    type: string
+    description: Document date
+    example: "2024-01-15"
+  metadata:
+    type: object
+    description: Additional metadata fields
+    additionalProperties: true
+    example:
+      department: Engineering
+      category: Technical

specs/api/components/schemas/common/ProcessingMetadata.yaml

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+type: object
+description: Processing metadata for library document processing
+properties:
+  flow:
+    type: string
+    description: Flow ID
+    example: my-flow
+  collection:
+    type: string
+    description: Collection identifier
+    example: default
+  status:
+    type: string
+    description: Processing status
+    enum: [pending, processing, completed, failed]
+    example: completed
+  timestamp:
+    type: string
+    format: date-time
+    description: Processing timestamp
+    example: "2024-01-15T10:30:00Z"
+  error:
+    type: string
+    description: Error message if processing failed
+    example: Failed to extract text from PDF

specs/api/components/schemas/common/RdfValue.yaml

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+type: object
+description: |
+  RDF value - represents either a URI/entity or a literal value.
+
+  When `e` is true, `v` must be a full URI (e.g., http://schema.org/name).
+  When `e` is false, `v` is a literal value (string, number, etc.).
+properties:
+  v:
+    type: string
+    description: The value - full URI when e=true, literal when e=false
+    example: http://example.com/Person1
+  e:
+    type: boolean
+    description: True if entity/URI, false if literal value
+    example: true
+required:
+  - v
+  - e
+example:
+  v: http://schema.org/name
+  e: true

specs/api/components/schemas/common/Triple.yaml

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+type: object
+description: |
+  RDF triple representing a subject-predicate-object statement in the knowledge graph.
+
+  Example: (Person1) -[has name]-> ("John Doe")
+properties:
+  s:
+    $ref: './RdfValue.yaml'
+    description: Subject - the entity the statement is about
+  p:
+    $ref: './RdfValue.yaml'
+    description: Predicate - the property or relationship
+  o:
+    $ref: './RdfValue.yaml'
+    description: Object - the value or target entity
+required:
+  - s
+  - p
+  - o
+example:
+  s:
+    v: http://example.com/Person1
+    e: true
+  p:
+    v: http://schema.org/name
+    e: true
+  o:
+    v: John Doe
+    e: false